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.
git-svn-id: https://develop.svn.wordpress.org/trunk@53091 602fd350-edb4-49c9-b593-d223f7449a82
