Migrate supports theme keys

[RobinMalfait]

This PR is a follow up of #18815 and #18816, but this time let's migrate the supports theme keys.

Let's imagine you have the following Tailwind CSS v3 configuration:

export default {
  content: ['./src/**/*.html'],
  theme: {
    extend: {
      supports: {
        // Automatically handled by bare values (using CSS variable as the value)
        foo: 'foo: var(--foo)', // parentheses are optional
        bar: '(bar: var(--bar))',

        // Not automatically handled because names differ
        baz: 'qux: var(--foo)',
   //   ^^^   ^^^       ← different names

        // Custom
        grid: 'display: grid',
      },
    },
  },
}

Then we would generate the following Tailwind CSS v4 CSS:

@custom-variant supports-baz {
  @supports (qux: var(--foo)) {
    @slot;
  }
}
@custom-variant supports-grid {
  @supports (display: grid) {
    @slot;
  }
}

Notice how we didn't generate a custom variant for data-foo or data-bar because those are automatically handled by bare values.

I also went with the longer form of @custom-variant, we could use the single selector approach, but that felt less clear to me.

@custom-variant supports-baz (@supports (qux: var(--foo)));
@custom-variant supports-grid (@supports (display: grid));

[RobinMalfait]

👋 bye bye blocked theme keys

We still have aria, data and supports handling in: https://github.com/tailwindlabs/tailwindcss/blob/feat%2Fupgrade-supports-theme/packages/tailwindcss/src/compat/theme-variants.ts

This is still necessary in case the config file could not be migrated due to other issues. So didn't get rid of that part.