Fixes the issue when you register blocks with `block.json` in your theme. There is no longer an assets's URL error because it resolves correctly in relation to the theme where it is located.
Props fabiankaegy, ocean90, whoisnegrello, audrasjb, peterwilsoncc,
Fixes#54647, #55513.
Built from https://develop.svn.wordpress.org/trunk@53091
git-svn-id: http://core.svn.wordpress.org/trunk@52680 1a063a9b-81f0-0310-95a4-ce76da25c4cd