Setup GitHub actions (#9)

* Setup GitHub actions PR pipeline that runs phpunit and phpcs

.github/workflows/pr.yml

@@ -0,0 +1,47 @@
+name: Image Comparator PR pipeline
+
+on:
+  pull_request:
+    types:
+      - assigned
+      - opened
+      - reopened
+      - review_requested
+      - synchronize
+
+jobs:
+  phpunit:
+    runs-on: ubuntu-24.04
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+
+      - name: Set up PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.2'
+
+      - name: Install Composer dependencies
+        run: composer install --prefer-dist --no-progress --no-suggest
+
+      - name: Run PHPUnit tests
+        run: ./vendor/bin/phpunit --testsuite=unit
+
+  phpcs:
+    runs-on: ubuntu-24.04
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+
+      - name: Set up PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.2'
+
+      - name: Install Composer dependencies
+        run: composer install --prefer-dist --no-progress --no-suggest
+
+      - name: Run PHP CodeSniffer
+        run: ./vendor/bin/phpcs -p .