Resolve theme keys when migrating JS config to CSS

[philipp-spiess]

With the changes in #14672, it now becomes trivial to actually resolve the config (while still retaining the reset behavior). This means that we can now convert JS configs that use functions, e.g.:

import { type Config } from 'tailwindcss'

export default {
  theme: {
    extend: {
      colors: ({ colors }) => ({
        gray: colors.neutral,
      }),
    },
  },
} satisfies Config

This becomes:

@import 'tailwindcss';

@theme {
  --color-gray-50: #fafafa;
  --color-gray-100: #f5f5f5;
  --color-gray-200: #e5e5e5;
  --color-gray-300: #d4d4d4;
  --color-gray-400: #a3a3a3;
  --color-gray-500: #737373;
  --color-gray-600: #525252;
  --color-gray-700: #404040;
  --color-gray-800: #262626;
  --color-gray-900: #171717;
  --color-gray-950: #0a0a0a;
}

[thecrypticace]

Looks good except for the test failures (color palette related). Fix those up and should be good to go.

[RobinMalfait]

Approved but some small changes

[RobinMalfait]

I know this PR didn't introduce it, but I wonder if the "legacy" colors (v3) should still use the v3 values (hex) instead of oklch(…) 🤔