Add GitHub Actions to automatically run tests

.github/workflows/ci.yml

@@ -0,0 +1,40 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+
+jobs:
+  phpunit:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+        php: [7.3, 7.4]
+        dependency-version: [prefer-lowest, prefer-stable]
+
+    name: P${{ matrix.php }} - ${{ matrix.dependency-version }} - ${{ matrix.os }}
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v1
+
+      - name: Cache dependencies
+        uses: actions/cache@v1
+        with:
+          path: ~/.composer/cache/files
+          key: dependencies-php-${{ matrix.php }}-composer-${{ hashFiles('composer.json') }}
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v1
+        with:
+          php-version: ${{ matrix.php }}
+          coverage: none
+
+      - name: Install dependencies
+        run: |
+          composer install
+          composer update --${{ matrix.dependency-version }} --prefer-dist --no-interaction --no-suggest
+
+      - name: Execute tests
+        run: vendor/bin/phpunit tests

.gitignore

@@ -1,5 +1,6 @@
 *
 !*/
+!/.github/**
 !/.gitignore
 !/composer.json
 !/src/**