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.

test/unit/node/routes/vscode.test.ts

@@ -103,7 +103,7 @@ describe("vscode", () => {
   it("should do nothing when nothing is passed in", async () => {
     codeServer = await integration.setup(["--auth=none"], "")
 
-    let resp = await codeServer.fetch("/", undefined)
+    const resp = await codeServer.fetch("/", undefined)
 
     expect(resp.status).toBe(200)
     const url = new URL(resp.url)