Add `prose`, `prose-<size>` and `prose-<color>` classes

[nokome]

This PR implements the prose, prose-<size> and prose-<color> classes of the Tailwind typography plugin (https://tailwindcss.com/docs/typography-plugin).

This is currently a draft PR as I'd like to get feedback on the approach I've taken before finishing it off (adding the non-default gray scales and sizes).

I haven't yet considered how to implement:

• https://tailwindcss.com/docs/typography-plugin#adapting-to-dark-mode, or
• https://tailwindcss.com/docs/typography-plugin#element-modifiers

[pintariching]

I'm not exactly sure how to implement plugins. Generally I think they shouldn't be tied to the core compiler but somehow separated so that you can have your own plugin without needing to rewrite the core logic.