Block Editor: Add predefined arrangements options support to InnerBlocks

[aduth]

Using InnerBlocks, it is currently possible to provide a template which describes the initial set of blocks which should be inserted in a new instance of that block:

https://github.com/WordPress/gutenberg/blob/master/packages/block-editor/src/components/inner-blocks/README.md#template

This should be enhanced to allow a user to select from a number of predefined, named template options.

Prior art:

When inserting a new Row block from the Coblocks plugin, the user is presented to choose both the number and from a number of layout options:

Usage examples:

• A Columns block could provide a few predefined layouts.
• A Contact Form block (like the one included in Jetpack) may provide options for specific types of forms (support request, mailing list signup).

Implementation proposal:

This could be considered a layer atop template, inheriting the same behaviors, but expressing a set of options from which the user can choose. Therefore, I would propose this would be named similarly, as a separate templateOptions prop.

It's necessary to consider what information is necessary for each option to be able to present it in the interface:

• Name (human-readable label)
• Icon
• (?) Defaultness

The Coblocks Row example is one more of a "wizard" of multiple steps to generate a block. It would be worth considering this, though also whether a more incremental or simple approach would be preferable.

Proposed usage ("Columns" as example):

<InnerBlocks
	templateOptions={ [
		{
			title: __( 'Two-thirds, one-third' ),
			icon: <svg { /* ... */ } />,
			template: [
				[ 'core/column', { width: 66.66 } ],
				[ 'core/column', { width: 33.33 } ],
			],
		},
		{
			title: __( 'Equal halves' ),
			icon: <svg { /* ... */ } />,
			template: [
				[ 'core/column', { width: 50 } ],
				[ 'core/column', { width: 50 } ],
			],
		},
		{
			title: __( 'Thirds' ),
			icon: <svg { /* ... */ } />,
			template: [
				[ 'core/column', { width: 33.33 } ],
				[ 'core/column', { width: 33.33 } ],
				[ 'core/column', { width: 33.33 } ],
			],
		},
	] }
/>	

Notes:

• Choice of title (vs. label, name, etc) is for consistency to block registration API