Re-enable Phoenix tests

This also runs the tests for contextlink, which I think we previously
never ran. I don't see any other packages that define an `npm test`
command. Having to manually list all the sub-package test directories in
the top-level package.json is a bit unfortunate, but it works.

For Puter itself we support Node 16.x IIRC, but Phoenix requires at
least 20.x currently.

.github/workflows/build-and-test.yml

@@ -0,0 +1,31 @@
+# This workflow will do a clean installation of node dependencies, cache/restore them, build the source code and run tests across different versions of node
+# For more information see: https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-nodejs
+
+name: Node.js CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        node-version: [20.x, 21.x]
+        # See supported Node.js release schedule at https://nodejs.org/en/about/releases/
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Use Node.js ${{ matrix.node-version }}
+      uses: actions/setup-node@v4
+      with:
+        node-version: ${{ matrix.node-version }}
+        cache: 'npm'
+    - run: npm ci
+    - run: npm run build --if-present
+    - run: npm test