Blocks: Introduce the block parser.

The `WP_Block_Parser` class, and the accompanying `parse_blocks()` helper function, can be used to parse an array of blocks out of a content string.

`WP_Block_Parser` is copied from the `@wordpress/block-serialization-default-parser` package. To ensure it stays in sync with the JavaScript parser, changes should be implemented in the package first, then the package version should be upgraded to include the changes.

Props pento.

Merges [43751] to trunk.

See #45109.
Built from https://develop.svn.wordpress.org/trunk@44116


git-svn-id: https://core.svn.wordpress.org/trunk@43946 1a063a9b-81f0-0310-95a4-ce76da25c4cd