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/lib.sh

@@ -14,7 +14,7 @@ pkg_json_version() {
 }
 
 vscode_version() {
-  jq -r .version vendor/modules/code-oss-dev/package.json
+  jq -r .version lib/vscode/package.json
 }
 
 os() {