Formatter: `allow_form_feeds` preview style

[MichaReiser]

Implement Black's allow_form_feeds style as a preview style in Ruff.

Black now retains form feed characters on module level on otherwise empty lines. Supporting this would be nice but adds some complexity because we need to check the source text to identify from feed characters.

I consider this a nice to have but not a must for the stable release of the formatter.

[MichaReiser]

This is a neat feature but rather nish. I'm happy to provide help if someone's interested in implementing this. I'll close this in the meantime to clarify that it is unplanned.