Add validate workflow (#369)

* add validate workflow

* make typo to test 3c61c88

* remove unneeded if

* fix typo

* better names

* Update validate-metadata-files.yml

.github/workflows/validate-metadata-files.yml

+name: "Validate devcontainer-feature.json files"
+on:
+  workflow_dispatch:
+  pull_request:
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      
+      - name: "Validate devcontainer-feature.json files"
+        uses: devcontainers/action@v1
+        with:
+          validate-only: "true"
+          base-path-to-features: "./src"