Blocks: Allow reading the script handle from asset files

In the [documentation for WPDefinedAsset definition](https://developer.wordpress.org/block-editor/reference-guides/block-api/block-metadata/#wpdefinedasset) for block metadata there is a note about the handle. That was missing in WordPress core.

Props gziolo, jsnajdr, youknowriad.
Fixes #60485.




git-svn-id: https://develop.svn.wordpress.org/trunk@57590 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/blocks/notice/shared-script.asset.php

@@ -0,0 +1,7 @@
+<?php
+
+return array(
+	'handle'       => 'tests-my-shared-script',
+	'dependencies' => array(),
+	'version'      => 'test',
+);

tests/phpunit/data/blocks/notice/shared-script.js

@@ -0,0 +1 @@
+/* Another test JavaScript file. */