Fix more failure cases of assistant edits (#19653)

* Make `description` optional (since we describe it as optional in the
prompt, and we're currently not showing it)
* Fix fuzzy location bug that neglected the cost of deleting prefixes of
the query.
* Make auto-indent work for single-line edits. Previously, auto-indent
would not occur when overwriting a single line (without inserting or
deleting a newline)

Release Notes:

- N/A

crates/languages/Cargo.toml

@@ -10,7 +10,7 @@ workspace = true
 
 [features]
 test-support = [
-    "tree-sitter"
+    "load-grammars"
 ]
 load-grammars = [
     "tree-sitter-bash",
@@ -82,3 +82,8 @@ text.workspace = true
 theme = { workspace = true, features = ["test-support"] }
 unindent.workspace = true
 workspace = { workspace = true, features = ["test-support"] }
+tree-sitter-typescript.workspace = true
+tree-sitter-python.workspace = true
+tree-sitter-go.workspace = true
+tree-sitter-c.workspace = true
+tree-sitter-css.workspace = true