Blocks: Introduce `WP_Block_Type` and `WP_Block_Type_Registry` classes.

These are the foundational classes allowing blocks to be registered and used throughout WordPress.

This commit also includes the `has_block()` and `has_blocks()` functions, which are required for unit testing these classes.

Merges [43742] from the 5.0 branch to trunk.

Props adamsilverstein, danielbachhuber, desrosj.
Fixes #45097.
See #45109.

Built from https://develop.svn.wordpress.org/trunk@44108


git-svn-id: https://core.svn.wordpress.org/trunk@43938 1a063a9b-81f0-0310-95a4-ce76da25c4cd