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/build/build-vscode.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-# Builds vscode into vendor/modules/code-oss-dev/out-vscode.
+# Builds vscode into lib/vscode/out-vscode.
 
 # MINIFY controls whether a minified version of vscode is built.
 MINIFY=${MINIFY-true}
@@ -9,7 +9,7 @@ MINIFY=${MINIFY-true}
 main() {
   cd "$(dirname "${0}")/../.."
 
-  cd vendor/modules/code-oss-dev
+  cd lib/vscode
 
   # Any platform works since we have our own packaging step (for now).
   yarn gulp "vscode-reh-web-linux-x64${MINIFY:+-min}"