Blocks: Add new `render` property in `block.json` for block types

[gziolo]

See comment from @spacedmonkey on GitHub (​https://github.com/WordPress/gutenberg/pull/13693#discussion_r287337504):

Current the render callback is a function call. How would it work as a file? Why not make it renderTemplate but still allow for renderCallback. Function calls, should be a method in a class or be namespaced, which wouldn't really work in another file.

ACF blocks, has a rendercallback / render template in it's block definitions.

There is also another thread started by @spacedmonkey in ​https://github.com/WordPress/gutenberg/pull/13693#discussion_r288121797 which expands on this topic.

[gziolo]

I created a prototype in the Gutenberg plugin: ​https://github.com/WordPress/gutenberg/pull/42430.

Trac ticket: https://core.trac.wordpress.org/ticket/53148

[gziolo]

In 54132:

Blocks: Add new render property in block.json for block types

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.

Committed with https://core.trac.wordpress.org/changeset/54132.

[SergeyBiryukov]

FWIW, I think core generally does not include DocBlocks for anonymous functions like the one added in [54132], but I don't see any issues with that at a glance. I guess it won't be displayed in the ​WordPress Developer Reference, just hope it does not break the parser :)

[gziolo]

I hope it doesn't break. Would it help if we add an additional PHPDoc keyword like @internal?