Add plugin schema validator workflow (#505)

* Remove code formatting workflows and related configuration files

* Remove formatting instructions from README

* Add plugin validator

* Add workflow

* Limit workflow to plugins/themes directories

.github/workflows/validate.yml

@@ -0,0 +1,24 @@
+name: Validate Plugins
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'plugins/**'
+      - 'themes/**'
+  pull_request:
+    branches: [main]
+    paths:
+      - 'plugins/**'
+      - 'themes/**'
+
+jobs:
+  validate:
+    runs-on: ubuntu-22.04
+    steps:
+    - uses: actions/checkout@v4
+    - uses: actions/setup-node@v4
+      with:
+        node-version: '20.x'
+    - run: cd ./validator && yarn install --frozen-lockfile
+    - run: node ./validate.js --ci