chore: move Code to a submodule (#4990)

* Move Code to a submodule

Closes #4901.

* Base Code cache on hash and re-enable node_modules cache

The current setup appears to only rebuild VS Code if the dependencies
change but we need to rebuild it if anything changes.

I also re-enabled the commented out node_modules caches.  They look like
they should work to me with the submodule method.  I think the problem
occurred because Code itself was being installed in the yarn step.

ci/dev/watch.ts

@@ -14,7 +14,7 @@ class Watcher {
   private rootPath = path.resolve(process.cwd())
   private readonly paths = {
     /** Path to uncompiled VS Code source. */
-    vscodeDir: path.join(this.rootPath, "vendor", "modules", "code-oss-dev"),
+    vscodeDir: path.join(this.rootPath, "lib/vscode"),
     pluginDir: process.env.PLUGIN_DIR,
   }