Add text-emphasis-* utilities

[olivermrose]

Adds utilities for:

• text-emphasis-style
• text-emphasis-color
• text-emphasis-position

The property for text-emphasis-style is text-emphasis to be consistent with text-decoration.

[lukewarlow]

I personally think it should use the long hand properties for all of them. Otherwise there's unintuitive override behaviour when using multiple properties together. Which someone's trying to fix for text-decoration here #6378

[adamwathan]

Hey thanks for the PR! This is quite a lot of API (and selfishly new documentation to write and maintain) for something that I think isn't very commonly used so I think for now I want to say no to this and recommend throwing it into a plugin if you want to reuse it across your projects.

Not opposed to adding it one day in the future but since this has been sitting open forever I feel like it's best to make a decision on it for now. Thanks either way I do appreciate it! ❤️

If I'm missing a really obvious and popular use case for this please create either a discussion or a new PR with some more details and can definitely revisit.