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/fmt.sh

@@ -19,7 +19,7 @@ main() {
     "*.sh"
   )
   prettier --write --loglevel=warn $(
-    git ls-files "${prettierExts[@]}" | grep -v "lib/vscode" | grep -v "vendor/modules/code-oss-dev" | grep -v 'helm-chart'
+    git ls-files "${prettierExts[@]}" | grep -v "lib/vscode" | grep -v 'helm-chart'
   )
 
   doctoc --title '# FAQ' docs/FAQ.md > /dev/null