Add vscode via vendor package.

- Use yarn for vscode vendoring.
- Grab hash from package.

vendor/postinstall.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+main() {
+  echo 'Installing VS Code dependencies...'
+  cd modules/code-oss-dev
+  yarn install --frozen-lockfile
+}
+
+main "$@"