feat: add coding standard action (#51)

coding-standard/README.md

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+# Magento 2 Coding Standard Action
+
+A Github Action that runs the Magento Coding Standard.
+
+## Inputs
+
+See the [action.yml](./action.yml)
+
+## Usage
+
+```yml
+name: Coding Standard
+
+on:
+  push:
+    branches:
+    - main
+  pull_request:
+    branches:
+    - main
+
+jobs:
+  coding-standard:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: graycoreio/github-actions-magento2/coding-standard@main
+      with:
+        version: 25 # Optional, will use the latest if omitted.
+        path: app/code # Optional, will be used when event is not a pull request.
+```

coding-standard/action.yml

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+name: "Coding Standard"
+author: "Graycore"
+description: "A Github Action that runs the Magento Coding Standard."
+
+inputs:
+  php_version:
+    required: true
+    default: "8.1"
+    description: "PHP version used to do the coding standard check."
+
+  composer_version:
+    required: true
+    default: "2"
+    description: "The version of composer to use."
+
+  path:
+    required: true
+    default: 'app/code'
+    description: "The directory (relative to the project root) in which the coding standard will be checked. Used when the event is not a pull request."
+
+  version:
+    required: false
+    description: "The version of the coding standard to use. If not provided, will use the latest version."
+
+runs:
+  using: composite
+  steps:
+    - name: Checkout Project
+      uses: actions/checkout@v3
+      with:
+        fetch-depth: 0
+        path: project
+
+    - name: Create Standard Directory
+      shell: bash
+      run: mkdir standard
+
+    - name: Set PHP Version
+      uses: shivammathur/setup-php@v2
+      with:
+        php-version: ${{ inputs.php_version }}
+        tools: composer:v${{ inputs.composer_version }}
+        coverage: none
+
+    - name: Install Coding Standard
+      shell: bash
+      working-directory: standard
+      run: composer require "magento/magento-coding-standard:${{ inputs.version || '*' }}"
+
+    - name: Register Coding Standard
+      shell: bash
+      working-directory: standard
+      run: vendor/bin/phpcs --config-set installed_paths ${{ github.workspace }}/standard/vendor/magento/magento-coding-standard,${{ github.workspace }}/standard/vendor/phpcompatibility/php-compatibility
+
+    - name: Get Changed Files
+      shell: bash
+      working-directory: project
+      id: changed-files
+      run: echo "::set-output name=files::$(git diff --name-only --diff-filter=ACMRT ${{ github.event.pull_request.base.sha }} ${{ github.sha }} | xargs)"
+      if: github.event_name == 'pull_request'
+
+    - name: Coding Standard Check
+      shell: bash
+      run: ../standard/vendor/bin/phpcs --standard=Magento2 ${{ github.event_name == 'pull_request' && steps.changed-files.outputs.files || inputs.path }}
+      working-directory: project