Migrate data theme keys

[RobinMalfait]

This PR is similar to and a follow up of #18815, but this time to migrate the data theme keys.

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

export default {
  content: ['./src/**/*.html'],
  theme: {
    extend: {
      data: {
        // Automatically handled by bare values
        foo: 'foo',
    //  ^^^   ^^^       ← same names

        // Not automatically handled by bare values
        bar: 'baz',
    //  ^^^   ^^^       ← different names

        // Completely custom
        checked: 'ui~="checked"',
      },
    },
  },
}

Then we would generate the following Tailwind CSS v4 CSS:

@custom-variant data-bar (&[data-baz]);
@custom-variant data-checked (&[data-ui~="checked"]);

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