Editor: Make asset registration .asset.php optional for blocks

[gziolo]

A related issue in Gutenberg: ​https://github.com/WordPress/gutenberg/issues/57234.

It is a hard requirement that a *.asset.php file is present to register a block in WordPress. Otherwise, it throws a Warning:

function register_block_script_handle was called incorrectly. The asset file (/wp-content/uploads/faustwp/blocks/block-a/index.asset.php) for the "editorScript" defined in "create-block/block-a" block definition is missing. (This message was added in version 5.5.0.)

and the block will not appear in the inserter.

Trac ticket: https://core.trac.wordpress.org/ticket/60460

Committed with https://core.trac.wordpress.org/changeset/57559.

Thanks for getting this over the line @gziolo and thanks @josephfusco for kicking it off 🙌