Editor: Introduce WP_Block and WP_Block_List classes
Backports functionality added in Gutenberg in the following PRs:
It's a few ideas related to block rendering and the provided block value, which is particularly impactful for work around block context.
Props aduth, TimothyBJacobs, noisysocks, epiqueras, youknowriad, talldanwp, zebulan. Fixes #49926.