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/main.ts

@@ -31,7 +31,7 @@ export const shouldSpawnCliProcess = (args: UserProvidedArgs): boolean => {
 export const runVsCodeCli = async (args: DefaultedArgs): Promise<void> => {
   logger.debug("Running VS Code CLI")
 
-  // See ../../vendor/modules/code-oss-dev/src/vs/server/node/server.main.js.
+  // See ../../lib/vscode/src/vs/server/node/server.main.js.
   const spawnCli = await loadAMDModule<CodeServerLib.SpawnCli>("vs/server/node/server.main", "spawnCli")
 
   try {