Blocks: Introduce WP_Block_Processor for efficiently parsing blocks.
The Block Processor follows the HTML API in providing a streaming, near-zero-overhead, lazy, re-entrant parser for traversing block structure. This class provides an alternate interface to parse_blocks() which is more amenable to a number of common server-side operations on posts, especially those needing to operate on only a part of a full post.
Developed in https://github.com/WordPress/wordpress-develop/pull/9105
Discussed in https://core.trac.wordpress.org/ticket/61401
Props dmsnell, gziolo, jonsurrell, soean, tjnowell, westonruter.
Fixes #61401.