Add GitHub workflow 'test'

Runs `composer install` and `phpcs` on basically every push, replaces Travis CI.

.github/workflows/test.yml

@@ -0,0 +1,98 @@
+name: Test Pico CMS
+
+on:
+    push:
+        branches:
+            - master
+            - pico-3.0
+        tags: [ 'v*.*.*' ]
+    pull_request:
+        branches:
+            - master
+            - pico-3.0
+
+jobs:
+    test:
+        name: PHP ${{ matrix.PHP_VERSION }}
+
+        runs-on: ubuntu-latest
+        permissions:
+            contents: read
+
+        strategy:
+            matrix:
+                PHP_VERSION:
+                    - '7.2'
+                    - '7.3'
+                    - '7.4'
+                    - '8.0'
+                    - '8.1'
+                    - 'nightly'
+            fail-fast: false
+
+        continue-on-error: ${{ matrix.PHP_VERSION == 'nightly' }}
+
+        env:
+            PHP_VERSION: ${{ matrix.PHP_VERSION }}
+
+        steps:
+            - name: Checkout repository
+              uses: actions/checkout@v2
+
+            - name: Setup PHP ${{ env.PHP_VERSION }}
+              uses: shivammathur/setup-php@v2
+              with:
+                  php-version: ${{ env.PHP_VERSION }}
+                  tools: composer, phpcs
+
+            - name: Read Pico version
+              run: |
+                  PICO_VERSION="$(php -r 'require("./lib/Pico.php"); echo Pico::VERSION;')"
+                  echo "PICO_VERSION=$PICO_VERSION" | tee -a "$GITHUB_ENV"
+
+            - name: Setup Composer
+              if: ${{ github.ref_type == 'branch' }}
+              run: |
+                  COMPOSER_ROOT_VERSION=
+
+                  COMPOSER_BRANCH_ALIAS="$(php -r "
+                      \$json = json_decode(file_get_contents('./composer.json'), true);
+                      if (\$json !== null) {
+                          if (isset(\$json['extra']['branch-alias']['dev-$GITHUB_REF_NAME'])) {
+                              echo 'dev-$GITHUB_REF_NAME';
+                          }
+                      }
+                  ")"
+
+                  if [ -z "$COMPOSER_BRANCH_ALIAS" ]; then
+                      PICO_VERSION_PATTERN="$(php -r "
+                          require_once('./lib/Pico.php');
+                          echo preg_replace('/\.[0-9]+-dev$/', '.x-dev', Pico::VERSION);
+                      ")"
+                  fi
+
+                  if [ -n "$PICO_VERSION_PATTERN" ]; then
+                      COMPOSER_ROOT_VERSION="$PICO_VERSION_PATTERN"
+                      echo "COMPOSER_ROOT_VERSION=$COMPOSER_ROOT_VERSION" | tee -a "$GITHUB_ENV"
+                  fi
+
+            - name: Get Composer cache directory
+              run: |
+                  COMPOSER_CACHE_DIR="$(composer config cache-dir)"
+                  echo "COMPOSER_CACHE_DIR=$COMPOSER_CACHE_DIR" | tee -a "$GITHUB_ENV"
+
+            - name: Restore Composer cache
+              uses: actions/cache@v2
+              with:
+                  path: ${{ env.COMPOSER_CACHE_DIR }}
+                  key: ${{ runner.os }}-composer-php${{ matrix.PHP_VERSION }}
+                  restore-keys: |
+                      ${{ runner.os }}-composer-
+
+            - name: Install Composer dependencies
+              run: |
+                  composer install
+
+            - name: Run PHP_CodeSniffer
+              run: |
+                  phpcs --standard=.phpcs.xml