ci: replace Travis with GitHub Actions workflow for PHP 8.1–8.5

.github/workflows/ci.yml

@@ -0,0 +1,43 @@
+name: CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+permissions:
+  contents: read
+
+jobs:
+  phpunit:
+    name: PHPUnit (PHP ${{ matrix.php-version }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        php-version:
+          - '8.1'
+          - '8.2'
+          - '8.3'
+          - '8.4'
+          - '8.5'
+    steps:
+      - name: Checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@f3e473d116dcccaddc5834248c87452386958240 # v2.37.2
+        with:
+          php-version: ${{ matrix.php-version }}
+          extensions: curl, json, mbstring
+          coverage: none
+
+      - name: Validate composer.json
+        run: composer validate --strict
+
+      - name: Install dependencies
+        run: composer install --prefer-dist --no-progress
+
+      - name: Run PHPUnit
+        run: vendor/bin/phpunit