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