Theme: Ensure theme styles are exposed via WordPress style handle

[aduth]

Task:

• A developer should be able to wp_enqueue_style( 'wp-theme' ); to enqueue design-tokens.css

Background:

Related comment: #73938 (comment)

We already treat @wordpress/theme as a "typical" script package in that we already have a script handle, and so it seems reasonable to have a style handle to bring in the required styles.

Implementation notes:

The build system looks for style.css for "auto-registration", so we may either need to ensure that a style.css is available for it to find, or update the build system to generalize styles exposed for auto-registration.

[ciampo]

Would the changes from #74743 suffice to close this issue as done?

[aduth]

Yes, I think #74743 satisfies this.