Extract C# support into an extension (#9971)

This PR extracts C# support into an extension and removes the built-in
C# support from Zed.

Tested using a Nix shell:

```
nix-shell -p dotnet-sdk omnisharp-roslyn
```

Release Notes:

- Removed built-in support for C#, in favor of making it available as an
extension. The C# extension will be suggested for download when you open
a `.cs` file.

extensions/csharp/languages/csharp/config.toml

@@ -0,0 +1,13 @@
+name = "CSharp"
+grammar = "c_sharp"
+path_suffixes = ["cs"]
+line_comments = ["// "]
+autoclose_before = ";:.,=}])>"
+brackets = [
+    { start = "{", end = "}", close = true, newline = true },
+    { start = "[", end = "]", close = true, newline = true },
+    { start = "(", end = ")", close = true, newline = true },
+    { start = "\"", end = "\"", close = true, newline = false, not_in = ["string"] },
+    { start = "'", end = "'", close = true, newline = false, not_in = ["string", "comment"] },
+    { start = "/*", end = " */", close = true, newline = false, not_in = ["string", "comment"] },
+]

extensions/csharp/languages/csharp/highlights.scm

@@ -0,0 +1,254 @@
+;; Methods
+(method_declaration name: (identifier) @function)
+(local_function_statement name: (identifier) @function)
+
+;; Types
+(interface_declaration name: (identifier) @type)
+(class_declaration name: (identifier) @type)
+(enum_declaration name: (identifier) @type)
+(struct_declaration (identifier) @type)
+(record_declaration (identifier) @type)
+(record_struct_declaration (identifier) @type)
+(namespace_declaration name: (identifier) @type)
+
+(constructor_declaration name: (identifier) @constructor)
+(destructor_declaration name: (identifier) @constructor)
+
+[
+  (implicit_type)
+  (predefined_type)
+] @type.builtin
+
+(_ type: (identifier) @type)
+
+;; Enum
+(enum_member_declaration (identifier) @property)
+
+;; Literals
+[
+  (real_literal)
+  (integer_literal)
+] @number
+
+[
+  (character_literal)
+  (string_literal)
+  (verbatim_string_literal)
+  (interpolated_string_text)
+  (interpolated_verbatim_string_text)
+  "\""
+  "$\""
+  "@$\""
+  "$@\""
+ ] @string
+
+[
+  (boolean_literal)
+  (null_literal)
+] @constant
+
+;; Comments
+(comment) @comment
+
+;; Tokens
+[
+  ";"
+  "."
+  ","
+] @punctuation.delimiter
+
+[
+  "--"
+  "-"
+  "-="
+  "&"
+  "&="
+  "&&"
+  "+"
+  "++"
+  "+="
+  "<"
+  "<="
+  "<<"
+  "<<="
+  "="
+  "=="
+  "!"
+  "!="
+  "=>"
+  ">"
+  ">="
+  ">>"
+  ">>="
+  ">>>"
+  ">>>="
+  "|"
+  "|="
+  "||"
+  "?"
+  "??"
+  "??="
+  "^"
+  "^="
+  "~"
+  "*"
+  "*="
+  "/"
+  "/="
+  "%"
+  "%="
+  ":"
+] @operator
+
+[
+  "("
+  ")"
+  "["
+  "]"
+  "{"
+  "}"
+]  @punctuation.bracket
+
+;; Keywords
+(modifier) @keyword
+(this_expression) @keyword
+(escape_sequence) @keyword
+
+[
+  "add"
+  "alias"
+  "as"
+  "base"
+  "break"
+  "case"
+  "catch"
+  "checked"
+  "class"
+  "continue"
+  "default"
+  "delegate"
+  "do"
+  "else"
+  "enum"
+  "event"
+  "explicit"
+  "extern"
+  "finally"
+  "for"
+  "foreach"
+  "global"
+  "goto"
+  "if"
+  "implicit"
+  "interface"
+  "is"
+  "lock"
+  "namespace"
+  "notnull"
+  "operator"
+  "params"
+  "return"
+  "remove"
+  "sizeof"
+  "stackalloc"
+  "static"
+  "struct"
+  "switch"
+  "throw"
+  "try"
+  "typeof"
+  "unchecked"
+  "using"
+  "while"
+  "new"
+  "await"
+  "in"
+  "yield"
+  "get"
+  "set"
+  "when"
+  "out"
+  "ref"
+  "from"
+  "where"
+  "select"
+  "record"
+  "init"
+  "with"
+  "let"
+] @keyword
+
+
+;; Linq
+(from_clause (identifier) @variable)
+(group_clause (identifier) @variable)
+(order_by_clause (identifier) @variable)
+(join_clause (identifier) @variable)
+(select_clause (identifier) @variable)
+(query_continuation (identifier) @variable) @keyword
+
+;; Record
+(with_expression
+  (with_initializer_expression
+    (simple_assignment_expression
+      (identifier) @variable)))
+
+;; Exprs
+(binary_expression (identifier) @variable (identifier) @variable)
+(binary_expression (identifier)* @variable)
+(conditional_expression (identifier) @variable)
+(prefix_unary_expression (identifier) @variable)
+(postfix_unary_expression (identifier)* @variable)
+(assignment_expression (identifier) @variable)
+(cast_expression (_) (identifier) @variable)
+
+;; Class
+(base_list (identifier) @type) ;; applies to record_base too
+(property_declaration (generic_name))
+(property_declaration
+  name: (identifier) @variable)
+(property_declaration
+  name: (identifier) @variable)
+(property_declaration
+  name: (identifier) @variable)
+
+;; Lambda
+(lambda_expression) @variable
+
+;; Attribute
+(attribute) @attribute
+
+;; Parameter
+(parameter
+  name: (identifier) @variable)
+(parameter (identifier) @variable)
+(parameter_modifier) @keyword
+
+;; Variable declarations
+(variable_declarator (identifier) @variable)
+(for_each_statement left: (identifier) @variable)
+(catch_declaration (_) (identifier) @variable)
+
+;; Return
+(return_statement (identifier) @variable)
+(yield_statement (identifier) @variable)
+
+;; Type
+(generic_name (identifier) @type)
+(type_parameter (identifier) @property)
+(type_argument_list (identifier) @type)
+(as_expression right: (identifier) @type)
+(is_expression right: (identifier) @type)
+
+;; Type constraints
+(type_parameter_constraints_clause (identifier) @property)
+
+;; Switch
+(switch_statement (identifier) @variable)
+(switch_expression (identifier) @variable)
+
+;; Lock statement
+(lock_statement (identifier) @variable)
+
+;; Method calls
+(invocation_expression (member_access_expression name: (identifier) @function))

extensions/csharp/languages/csharp/injections.scm

@@ -0,0 +1,2 @@
+((comment) @injection.content
+  (#set! injection.language "comment"))

extensions/csharp/languages/csharp/outline.scm

@@ -0,0 +1,38 @@
+(class_declaration
+    "class" @context
+    name: (identifier) @name
+) @item
+
+(constructor_declaration
+    name: (identifier) @name
+) @item
+
+(property_declaration
+    type: (identifier)? @context
+    type: (predefined_type)? @context
+    name: (identifier) @name
+) @item
+
+(field_declaration
+    (variable_declaration) @context
+) @item
+
+(method_declaration
+    name: (identifier) @name
+    parameters: (parameter_list) @context
+) @item
+
+(enum_declaration
+    "enum" @context
+    name: (identifier) @name
+) @item
+
+(namespace_declaration
+    "namespace" @context
+    name: (qualified_name) @name
+) @item
+
+(interface_declaration
+    "interface" @context
+    name: (identifier) @name
+) @item