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.
git-svn-id: https://develop.svn.wordpress.org/trunk@44108 602fd350-edb4-49c9-b593-d223f7449a82
Gary Pendergast