CI: switch to GitHub Actions - step 1: code style

This commit:
* Adds a GH Actions workflow for the CI code style check and to validate the `composer.json` file.
* Removes those actions from the `.travis.yml` configuration.
* Adds the new `.github` directory to the `.gitattributes` file to be `export-ignore`d.
* Adds a "Build Status" badge in the Readme to use the results from this particular GH Actions run.

Notes:
1. Builds will run on all pushes and on pull requests.
2. Builds can also be manually triggered.
    Note: manual triggering of builds has to be [explicitly allowed](https://github.blog/changelog/2020-07-06-github-actions-manual-triggers-with-workflow_dispatch/). This is not a feature which is enabled by default.
3. Composer dependency downloads will be cached for faster builds using a [predefined GH action](https://github.com/marketplace/actions/install-composer-dependencies) specifically created for this purpose.
    The alternative would be to handle the caching manually, which would add three extra steps to the script.
    Note: Caching works differently between Travis and GH Actions.
    On GH Actions, once a cache has been created, it can't be updated. It can only be replaced by a new cache with a different key.
    As the PHP version, the `composer.json` and a potential `composer.lock` hash are all part of the key used by the above mentioned action, this difference should not have a significant impact.
    Ref: https://docs.github.com/en/actions/advanced-guides/caching-dependencies-to-speed-up-workflows
4. The CS check will display the results in the actions script output log, as well as display any violations found inline in the GitHub code view using the [cs2pr](https://github.com/staabm/annotate-pull-request-from-checkstyle) tool.

Differences with the Travis implementation:
* This check will now run on ALL pushes and pulls.
    The branch filtering which was previously being applied in Travis has not been implemented for this script.
* The `composer validate` command will now only be run against the PHP version used in the `cs` script (PHP 7.4), not against multiple PHP versions.

.gitattributes

@@ -13,6 +13,7 @@
 phpcs.xml export-ignore
 phpcs.xml.dist export-ignore
 /.cache export-ignore
+/.github export-ignore
 
 #
 # Auto detect text files and perform LF normalization

.github/workflows/cs.yml

@@ -0,0 +1,44 @@
+name: CS
+
+on:
+  # Run on all pushes and on all pull requests.
+  push:
+  pull_request:
+  # Allow manually triggering the workflow.
+  workflow_dispatch:
+
+jobs:
+  checkcs:
+    name: 'Check code style'
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '7.4'
+          coverage: none
+          tools: cs2pr
+
+      # Validate the composer.json file.
+      # @link https://getcomposer.org/doc/03-cli.md#validate
+      - name: Validate Composer installation
+        run: composer validate --no-check-all
+
+      # Install dependencies and handle caching in one go.
+      # @link https://github.com/marketplace/actions/install-composer-dependencies
+      - name: Install Composer dependencies
+        uses: ramsey/composer-install@v1
+
+      # Check the codestyle of the files within YoastCS.
+      # The results of the CS check will be shown inline in the PR via the CS2PR tool.
+      # @link https://github.com/staabm/annotate-pull-request-from-checkstyle/
+      - name: Check PHP code style
+        continue-on-error: true
+        run: composer check-cs -- --report-full --report-checkstyle=./phpcs-report.xml
+
+      - name: Show PHPCS results in PR
+        run: cs2pr ./phpcs-report.xml

.travis.yml

@@ -18,14 +18,12 @@ branches:
 
 php:
   - 8.0
+  - 7.4
   - 7.0
   - 5.6
 
 jobs:
   fast_finish: true
-  include:
-    - php: 7.4
-      env: PHPCS=1
 
 before_install:
   - phpenv config-rm xdebug.ini || echo 'No xdebug config.'
@@ -41,10 +39,3 @@ install:
 script:
 # PHP Linting
 - composer lint
-
-# Validate the composer.json file.
-# @link https://getcomposer.org/doc/03-cli.md#validate
-- if [[ $TRAVIS_PHP_VERSION == "5.6" || $TRAVIS_PHP_VERSION == "7.4" ]]; then composer validate --no-check-all; fi
-
-# Check the code against YoastCS.
-- if [[ $PHPCS == "1" ]]; then composer check-cs; fi

README.md

@@ -1,3 +1,4 @@
+[![CS](https://github.com/Yoast/i18n-module/actions/workflows/cs.yml/badge.svg)](https://github.com/Yoast/i18n-module/actions/workflows/cs.yml)
 [![Build Status](https://travis-ci.org/Yoast/i18n-module.png?branch=master)](https://travis-ci.org/Yoast/i18n-module)
 [![Code Climate](https://codeclimate.com/github/Yoast/i18n-module/badges/gpa.svg)](https://codeclimate.com/github/Yoast/i18n-module)