Add "Style variations" theme tag

[poena]

Add a new theme tag called "Style variations" to help users find themes with style variations, a block theme feature where the theme developer includes an alternative .json configuration file for block settings and styles.

• Theme developers can add the tag 'style-variations' to the file header in style.css.
• Users can sort themes with style variations using the theme filter on the theme selection screen.

Companion ticket to ​https://meta.trac.wordpress.org/ticket/6545.

[mukesh27]

@poena Small change can you align the arrow in the patch?

[robinwpdeveloper]

Do we need to do any testing for this?
If yes, @poena can you provide testing instructions? :)

[costdev]

Adjust capitalization for consistency with the other strings.

[costdev]

This ticket was discussed in the recent bug scrub. It was agreed that the Style variations string should have its capitalization adjusted to Style Variations for consistency with the other strings. I've added 56869-3.diff which does this.

Adding the commit keyword.

This PR refreshes patch: 41574.2.diff to:

• Fix CS issues.
• Update @since annotation and remove an empty line from the docblock.
• Backward compatibility: Use single quotes for attribute values.
• Remove an unnecessary array_map() for performance.
• Addresses concerns about the image attributes and link attributes clashing.
  • Image attributes: Stays as $attr for backward compatibility (i.e. named parameters).
  • Link attributes: Is called $attributes and, once collapsed to a string, $link_attributes, for consistency with $link_text.
• Add tests.

Trac ticket: https://core.trac.wordpress.org/ticket/56869

[costdev]

Mis-link of PR, sincere apologies. 56869-3.diff is still a valid patch for this ticket.

[SergeyBiryukov]

In 55149:

Themes: Add "Style Variations" to the list of WordPress theme features.

Follow-up to ​[meta12372].

Props poena, mukesh27, costdev.
Fixes #56869. See ​#meta6545.

committed in https://core.trac.wordpress.org/changeset/55262