Add markdown validation to the readme validator

[zodiac1978]

There is a Readme validator on wordpress.org:
​https://wordpress.org/plugins/developers/readme-validator/

But this is only checking for the readme standard:
​https://developer.wordpress.org/plugins/wordpress-org/how-your-readme-txt-works/

And not for the markdown.

If a plugin is for example omitting the “closing” of the title markdown, writing = V3.9.2 - 04.07.2021 instead of = V3.9.2 - 04.07.2021 = the validator wouldn't mark this with an error or warning or note.

Although it would help to show a preview (see: #2538), maybe we could validate markdown additionally.

There are some PHP markdown parser, but I am not sure if there are validators (or if those parser can be used for validating):
​https://php.watch/articles/php-markdown-libraries

If we have a plugin/library for this it could be used in Github actions or other test environments before deployment to wordpress.org. Or we could maybe add this check to the pre-commit hook, so we can reject committing wrong formatted Readmes.

Thanks to @dd32, @clorith an @ocean90 for the help on Slack about this issue.