Add layered shadow support, add support for inset shadows (non-breaking changes)

[drwpow]

Summary

This proposes the following changes to $type: shadow tokens:

• Allows arrays for $value
• Adds inset (boolean) property

Reasoning

This PR proposes that the following discussions be resolved like so:

• Shadow type feedback #100: ✅ Accepted (all proposals that I saw)

Pros

• Adds layered shadow support (example) without breaking existing shadow tokens
• Adds support for inset shadows, also without breaking backwards compatibility

Cons

• Arrays vs non-arrays adds a minor bit of tooling annoyance, but nothing significant

Alternatives

• We could just enforce “always arrays,” but IMO that’s not necessary to put that burden on the people writing JSON. The spec allows for other “syntactic sugar” and this seems like a good QoL addition
• We could make inset required, at the expense of breaking backward compatibility
• Instead of "inset": [boolean], we could have something like "position": "inner" (string enum), but I can’t think of any other value this would be (what is there other than “inside” and “outside”?). Seems like designing for something no one has asked for.

Notes

• Other token types such as gradients and cubic béziers already have arrays for values, so this doesn’t introduce inconsistencies in the design

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[kaelig]

This proposal looks good to me, we'll make adjustments to the color values separately shortly.