Blocks: Introduce WP_Block_Scanner for efficiently parsing blocks.

The Block Scanner 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, such as:

 - Generating an excerpt from only the first N blocks in a post.
 - Determining which block types are present in a post.
 - Determining which posts contain a block of a given type.
 - Generating block supports content for a post.

Author: dmsnell