New `render` field in `block.json` file that accepts a string value. It allows to pass a path to the PHP file that is going to be used to render the block on the server. Related PR in Gutenberg: https://github.com/WordPress/gutenberg/pull/42430.
Props spacedmonkey, luisherranz, welcher, noisysocks, matveb, fabiankaegy, aristath, zieladam.
Fixes#53148.
git-svn-id: https://develop.svn.wordpress.org/trunk@54132 602fd350-edb4-49c9-b593-d223f7449a82
