Add outlines for CSS (#8706)

Add outlines for .css files

⚠️I also added `workspace = { workspace = true, features =
["test-support"] }` to dev deps of 'languages' crate to make unit tests
work.

![Css
outlines](https://github.com/zed-industries/zed/assets/4057095/0cade407-79e7-4d0f-9b80-3502509e373d)

crates/languages/src/css/outline.scm

@@ -0,0 +1,18 @@
+(stylesheet
+    (import_statement
+        "@import" @context
+        ((string_value) @name)) @item)
+
+
+(rule_set
+    (selectors
+      .
+      (_) @name
+      ("," @name (_) @name)*
+    )) @item
+
+(media_statement
+    "@media" @context
+    (_) @name
+    (block)
+) @item