Add text-shadow-* utilities

[philipp-spiess]

This PR adds new text-shadow-* utilities and default values courtesy of @danhollick's.

Usage is similar to the normal shadow-* utilities, for example:

<h1 class="text-center text-7xl tracking-tight text-white text-shadow-xl">
  Some fancy <br />
  headline
</h1>

Since this PR also adds first-class support for the --text-shadow theme namespace, it also means it resolves #17047.

Test plan

• Copied @danhollick's demo playground to the vite setup and ran a production build to ensure it looks correct: https://tailwind-text-shadows-preview-2kdnjb32b.vercel.app/

[danhollick]

@philipp-spiess I think you might have used an old play link, which is my bad. I updated the link in trello but not in discord. (New link)

I readjusted the scale so it goes from 2xs to lg to match the box-shadow one. Same values, just all shifted down one.

[philipp-spiess]

@danhollick Ah great spot, updated it and also ran your play through a Vite project to make sure it's creating the right CSS: https://tailwind-text-shadows-preview-2kdnjb32b.vercel.app/

[adamwathan]

@philipp-spiess Can we add UI tests for this one to make sure the shadow colors work? Like the box-shadow utilities we have to not really use the variables here annoyingly for the colors to work, so if we ever forget that and try to refactor it to use variables it would be good to have tests that fail.