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/test-unit.sh

@@ -14,7 +14,7 @@ main() {
   # Our code imports from `out` in order to work during development but if you
   # have only built for production you will have not have this directory.  In
   # that case symlink `out` to a production build directory.
-  local vscode="vendor/modules/code-oss-dev"
+  local vscode="lib/vscode"
   local link="$vscode/out"
   local target="out-build"
   if [[ ! -e $link ]] && [[ -d $vscode/$target ]]; then