Add text shadow props

[EvanGruhlkey]

What changed

• add a six-step --text-shadow-{1-6} scale
• include the new props in adaptive, light, dark, JavaScript, and resolver outputs
• document text-shadow usage on the project website
• add regression coverage for CSS and camel-cased JavaScript exports

Closes #588

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[argyleink]

excellent start! really only looking for a couple small things, thanks for hacking on this 🤘🏻

[argyleink]

the current demo isnt showcasing the work enough! here's your current demo page:

which i dont think was your intention. the suggestions i made to the demo help it become:

which is closer?

[argyleink]

ty, a new release will be available soon and your work will be inside!