Add GitHub Actions CI workflow

This workflow should ensure that the project is installable and the
Composer configuration is valid.

.github/workflows/ci.yml

@@ -0,0 +1,48 @@
+name: CI
+
+on:
+  push:
+    branches: [ main]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  tests:
+    name: Tests
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        env:
+          - AUTOLOAD=0
+          - AUTOLOAD=1
+        php-version:
+          - "7.4"
+          - "8.0"
+          - "8.1"
+
+    steps:
+      - uses: actions/checkout@master
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@master
+        with:
+          php-version: ${{ matrix.php-version }}
+          coverage: xdebug
+
+      - name: Get Composer Cache Directory
+        id: composer-cache
+        run: |
+          echo "::set-output name=dir::$(composer config cache-files-dir)"
+      - uses: actions/cache@v2
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('composer.json') }}
+
+      - name: Validate composer.json and composer.lock
+        run: composer validate
+
+      - name: Install dependencies
+        run: composer install --prefer-dist --no-progress --no-suggest