Dart support (#7220)

This is my first contribution, feedback is welcome.

Release Notes:

- Added Dart language support
([#5343](https://github.com/zed-industries/zed/issues/5343)).

crates/zed/src/languages/dart/outline.scm

@@ -0,0 +1,18 @@
+(class_definition
+    "class" @context
+    name: (_) @name) @item
+
+(function_signature
+    name: (_) @name) @item
+
+(getter_signature
+    "get" @context
+    name: (_) @name) @item
+
+(setter_signature
+    "set" @context
+    name: (_) @name) @item
+
+(enum_declaration
+    "enum" @context
+    name: (_) @name) @item