ci: add workflow to run wp plugin checks

[stklcode]

Add a new workflow to run the plugin-check-action when pushing or merging to stable.

We build the plugin first and re-use the .distignore file to generate the final plugin package. Otherwise we should run into several issues with files. This list is also used for deployment, so the results should be as representative as it gets.

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Run on current develop branch shows one error:

Error: The Stable Tag in your readme file does not match the version in your main plugin file. Your Stable Tag is meant to be the stable version of your plugin, not of WordPress. For your plugin to be properly downloaded from WordPress.org, those values need to be the same. If they’re out of sync, your users won’t get the right version of your code.

That's correct and intentional on the develop branch. Hence we should start running the full set of checks only before merging to stable.

and a few warnings

Warning: One or more tags were ignored. Please limit your plugin to 5 tags.

(now fixed, see #293)

Warning: Use of a direct database call is discouraged.
Warning: Direct database call without caching detected. Consider using wp_cache_get() / wp_cache_set() or wp_cache_delete().

(already known from PHPCS)

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[stklcode]

Forgot about this PR, the check workflow is now active (f0c3a40)