Update to allow alternative Blocks to handle Grouping interactions

[getdave]

This PR will allow alternative Blocks to be used to handle Grouping interactions. Currently, you can only use core/group but this PR will make it possible to register any valid Block as the Block to use when Grouping.

Previously the method that determined whether a given Block was the block to be used for “Grouping” interactions was hardcoded as “core/group. This PR builds upon #15774 to allow alternative Blocks to be used to handle the Grouping interactions.

This should address some of the concerns raised in #14908 (comment), namely:

• Avoid hardcoding refs to core/group within packages/blocks/src/api/factory.js
• Allow other Blocks to be registered to handle Grouping if required

Screenshots

Below shows me registering the test/alternative-group-block Block from a custom e2e testing Plugin as the default Grouping Block. I'm then using it within the Grouping interactions.

How has this been tested?

• Unit tests
• e2e tests
• Manual testing using the method outlined below.

How to test

• Open wp-admin in the Gutenberg docker env
• Activate the Gutenberg Test Custom Grouping Block Plugin (important: be sure to deactivate this after you have finished testing!)
• Create a new Page and add some Blocks to be Grouped.
• In the browser console
  • type window.wp.blocks.getGroupingBlockName() to get the current Grouping Block name - it should be core/group (the default).
  • type wp.blocks.setGroupingBlockName( 'test/alternative-group-block' ); - it should register that block (provided by the Plugin activated above) as the default for Grouping
• using the Editor UI
  • "Group" the Blocks using the "Group" item from the dropdown menu
  • Ungroup the Blocks again
  • Group the Blocks using the "Transform" control by selecting the "Alternative Group Block"

Questions

• Is relying on getGroupingBlockName() from the package here appropriate? I'm concerned I'm not using a select() here. Now uses select().
• Should we have some kind of validation in place before we allow custom blocks to be set as the Grouping Block? At the moment you can register any old Block but only Blocks which support the necessary transforms and utilise InnerBlocks are allowed. For example, you could try and register coblocks/column to act as the Grouping Block (it supports InnerBlocks after all) but within the correct convert() (transform) definition it won't function as you'd expect.
• Where should I write documentation to cover this feature (if at all)?

Types of changes

New feature (non-breaking change which adds functionality).

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[getdave]

@aduth @gziolo This PR is now passing all builds and is "ready to go" feature wise. However it still needs another tech review and manual test. Much appreciated.

[aduth]

Should this other reference be updated as well?

gutenberg/packages/block-editor/src/components/block-actions/index.js

Lines 120 to 121 in f51f662

	// Activate the `transform` on `core/group` which does the conversion
	const newBlocks = switchToBlockType( blocks, 'core/group' );

Less important, comment updates:

gutenberg/packages/blocks/src/api/factory.js

Line 150 in f51f662

// Don't allow single 'core/group' blocks to be transformed into

gutenberg/packages/blocks/src/api/factory.js

Line 377 in f51f662

// Unless it's a `core/group` Block then for multi block selections

gutenberg/packages/editor/src/components/convert-to-group-buttons/convert-button.js

Line 102 in f51f662

// Activate the `transform` on `core/group` which does the conversion

[getdave]

@aduth I've addressed those CR points.

I have no clue why the PHP unit tests have suddenly started failing...

[aduth]

Another potentially useful consequence of these functions is to better accommodate scenarios where the grouping block isn't available (either because it's not been registered or in allowing to explicitly unset by setGroupingBlockName( null )).

[getdave]

Thanks @aduth 👍