Add GitHub action to lint code (#19)

Implements a GitHub action to lint code automatically. This aims to further improve code quality by not allowing code with linting errors to be merged.

.github/workflows/wpcs.yml

@@ -0,0 +1,17 @@
+name: Lint the codebase
+
+on: pull_request
+
+jobs:
+  phpcs:
+    name: Lint
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: php-actions/composer@v6
+      with:
+        php_version: 7.0
+    - uses: php-actions/composer@v6
+      with:
+        php_version: 7.0
+        command: run lint