ci: add workflow to run wp plugin checks

[stklcode]

Branch name prefix "release/" was chosen just to trigger the workflow.

Current findings, just one (expected) error and some minor warnings.

Type	Description	Comment
Error	Tested up to: 5.7 < 6.7	should be updated before next release
Warning	Please limit your plugin to 5 tags	should be updated, no blocker
Warning	"Plugin URI" header not valid	should not point to w.org
Warning	$in_footer is not set explicitly wp_enqueue_script
Warning	Processing form data without nonce verification	not critical, just an empty($_GET...) check