Add gap-normal, gap-x-normal, and gap-y-normal utilities

[MichaelAllenWarner]

This PR adds gap-normal, gap-x-normal, and gap-y-normal utilities. Note that what the value of normal means for gap depends on whether the context is multi-column layout or grid/flex, but in either case it corresponds to the initial value.

Not sure how often this one would really be useful in practice, but since you've been adding these kinds of utilities recently (#10645, #10560) I figured I'd submit it.

[adamwathan]

Hey @MichaelAllenWarner thank you! I think going to pass on this by default for now just because they feel like weird values to include as part of your design system, especially when gap stuff usually needs to be tied to your spacing scale for consistency. Can always do this with arbitrary values or by customizing your config of course if it makes sense for your project 👍

Thank you either way, appreciate it as always!