Improve customizability and v3 color support

[adamwathan]

This PR is too big.

• Makes all previous color modifiers (prose-red) explicit entries in the theme with no magic (minor breaking change)
• Adds new color modifiers for the v3 alternate grays (prose-slate, prose-gray, prose-zinc, prose-neutral, prose-stone) to make it easy to style your typography to match the grays you are using on the rest of your site
• Adds new prose-invert class for easy dark mode support (prose dark:prose-invert)
• Abstracts colors behind variables to make adding a custom color modifier simpler
• Drops use of pseudo-elements in ul and ol elements for native ::marker pseudo (breaking change for anyone customizing bullets/counters using the old selectors)
• Adds a suite of element variants for targeting prose children, like prose-headings:text-black and prose-img:rounded-lg
• Rips out all of the old snapshot tests for simpler, more direct tests

I wish this PR were smaller so I had the motivation to write more about these changes but you'll just have to look at the tests or wait for updated documentation/release notes 🙊

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