feat(coding-standard)!: remove pr-diff feature and built-in php setup (#224)

BREAKING CHANGE: Much of the "setup" that's built-into the command is removed in favor of a leaner action. This also includes the "on PR, only diff PR contents" behavior. This can be restored, but it shouldn't be the default and should be done as an input.

Fix SEVERITY_FLAGS construction which exited 1 under bash -e when
severity inputs were empty, causing CI failures.

.github/workflows/_internal-coding-standard.yaml

@@ -51,9 +51,14 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
+
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          tools: composer:v${{ matrix.composer }}
+          coverage: none
+
       - uses: './coding-standard'
         with:
           version: ${{ github.event.inputs.version || '*' }}
-          path: ${{ github.event.inputs.path || '_test/demo-package' }}
-          composer_version: ${{ matrix.composer }}
-          php_version: ${{ matrix.php }}
+          path: ${{ github.event.inputs.path || '_test/demo-package' }}