Restore extensions in core (#114921)

* bring back grammar extensions

* fix hygiene

* Revert "pull themes from the marketplace"

This reverts commit 0738f76dac.

extensions/css/.vscode/launch.json

@@ -0,0 +1,14 @@
+{
+  "version": "0.2.0",
+  "configurations": [
+    {
+      "name": "Launch Grammar",
+      "type": "extensionHost",
+      "request": "launch",
+      "runtimeExecutable": "${execPath}",
+      "args": [
+        "--extensionDevelopmentPath=${workspaceRoot}"
+      ]
+    }
+  ]
+}