This commit introduces a new API to allow plugins to easily register block templates with `wp_register_block_template()` and the `WP_Block_Templates_Registry` class, addressing the complexity of hooking into multiple filters. It also ensures plugin-registered templates overridden by themes fall back to the plugin-provided title and description when the theme doesn't define them. See https://github.com/WordPress/gutenberg/pull/61577. See https://github.com/WordPress/gutenberg/pull/64610. Fixes #61804. Props aljullu, peterwilsoncc, antonvlasenko, azaozz, youknowriad, noisysocks. Built from https://develop.svn.wordpress.org/trunk@59073 git-svn-id: http://core.svn.wordpress.org/trunk@58469 1a063a9b-81f0-0310-95a4-ce76da25c4cd