Feat (Video Popup): add video schema

[kaeizen]

fixes #3402

Also added description in the schema to prevent "non-critical issues" flag in the validation.

[github-actions]

🤖 Pull request artifacts

file	commit
pr3465-stackable-3465-merge.zip	61fd584

[github-actions]

Try this Pull Request in the WordPress playground: https://playground.wordpress.net/?mode=seamless#{"landingPage":"/wp-admin/post-new.php?post_type=page","preferredVersions":{"php":"latest","wp":"latest"},"steps":[{"step":"login","username":"admin","password":"password"},{"step":"installPlugin","pluginZipFile":{"resource":"url","url":"https://raw.githubusercontent.com/gambitph/Stackable/artifacts/pr3465-stackable-3465-merge.zip"}}]}

[bfintal]

Looks like this is adding extra styles tot he datetime picker. Instead of explicitly adding the class names for the countdown and video popup blocks, can we style the datetime component when it's used in the Stackable block inspector in general? Please move these styles for the datetime outside the countdown editor.scss into somewhere more proper.