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.

src/node/routes/vscode.ts

@@ -118,7 +118,7 @@ export class CodeServerRouteWrapper {
     const { args } = req
 
     /**
-     * @file ../../../vendor/modules/code-oss-dev/src/vs/server/node/server.main.js
+     * @file ../../../lib/vscode/src/vs/server/node/server.main.js
      */
     const createVSServer = await loadAMDModule<CodeServerLib.CreateServer>("vs/server/node/server.main", "createServer")