Add allowedBlocks and TemplateLock attributes in Cover Block

[ajotka]

Description

According to #31312 request, I added allowedBlocks attribute for Cover Block.

How has this been tested? And Screenshots

Tested manually.

• Without allowedBlocks (before changes & after changes with Cover without passing allowedBlocks attribute)
• Expectations: Possibility to add all blocks

• With allowedBlocks = [ 'core/paragraph', 'core/heading', 'core/button' ]

• Expectations: Possibility to add only Paragraph, Heading and/or Button
  

• With templateLock

• Expectations: No 'add block' button and no possibility to add block
  

Types of changes

1. Add allowedBlocks and templateLock attributes with their types to block.json
2. Add allowedBlocks and templateLock (default false) attributes to edit.js
3. Add allowedBlocks and templateLock attributes to edit.native.js

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[github-actions]

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[jorgefilipecosta]

Setting a default here explicitly removes the locking even if the attribute is unset causing a behavior change. I think we should not include the default here.

[ajotka]

Same default is in Column PR: https://github.com/WordPress/gutenberg/pull/25778/files
Maybe there is no problem, but I prefer to inform you.

[ajotka]

The default value has been removed.

[mkaz]

This looks good.

I tested using a block filter to restrict what could be added.

wp.hooks.addFilter(
    'blocks.registerBlockType',
    'mkaz/testing-cover',
	function( settings, name ) {
		if ( name != 'core/cover') {
			return settings;
		}

		return {
			...settings,
			attributes: {
				...settings.attributes,
				allowedBlocks: {
					type: "array",
					default: [ 'core/paragraph', 'core/image' ],
				},
			}
		}
	}
);

This worked as expected with and without a templateLock value.

Thanks for the contribution! 👍

[github-actions]

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Kudos!

[Soean]

I think this is just a quick fix for the cover block. This feature is also useful for many other blocks. Maybe we should add it automatically via PHP to the Block API?