Add code style check on changed files only.

[message-dimke]

Changes proposed in this Pull Request:

Adding PHP multi-version code style check on PR files only.

Detailed test instructions:

There are two ways to test the PR.

Checkout test/phpcs-changed branch. Yes, it is not a typo #69.

1. Checkout test/phpcs-changed. This is the branch made from add/phpcs-changed but with few updates to make some checks fail.
2. Look into /src/phpcs_test folder for files with different code fragments supported by PHP8.2 but not PHP7.4 language versions.
3. Also there is the update to the workflow file not to fail-fast when running PR checks. Because of it you will see tests for 8.2 pass and 7.4 fail with reasons.
4. phpcs-changed is configured to look for changes in PR files against develop branch. Please note the command must look something like this:

vendor/bin/phpcs-changed --warning-severity=0 -s --git --git-base 0b1850098265bda5f2e38529b8651802804f21e8 src/phpcs_test/class-base.php src/phpcs_test/class-demo.php src/phpcs_test/misc.php

Note the list of files to check.

Also, please, not the --warning-severity=0 which will hide warnings.

Branch from add/phpcs-changed branch and use the instructions below.

1. Branch from add/phpcs-changed.
2. Update .github/workflows/php-cs-on-changes.yml with fail-fast: false property value.

    ...
    strategy:
      matrix:
        php: [7.4, 8.2]
      fail-fast: false
    ...

3. Download files into /src and commit them to your branch. phpcs_test.zip
4. Create a PR and see the checks.
5. Add changes to the files as will to compatibility check cases you know.

Changelog entry

Dev - Add phpcs on changed files only.