Block Editor: Add the Block Patterns API.

Backport WP_Block_Pattern_Categories_Registry, WP_Block_Patterns_Registry
and block patterns registration functions from the Gutenberg repository.

Props ocean90, ellatrix
Fixes #50445.

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git-svn-id: https://core.svn.wordpress.org/trunk@47925 1a063a9b-81f0-0310-95a4-ce76da25c4cd