Adds support for the new selectors property for block types. It adds it to the allowed metadata when registering a block type, makes the WP_Block_Type class aware of it, exposes it through the block types REST API, and the get_block_editor_server_block_settings function.
Corresponding work in the Gutenberg plugin: https://github.com/WordPress/gutenberg/pull/46496.
Fixes#57585.
Props aaronrobertshaw, hellofromTonya.
git-svn-id: https://develop.svn.wordpress.org/trunk@55673 602fd350-edb4-49c9-b593-d223f7449a82