Add spacing presets for theme support

[t-hamano]

What problem does this address?

#41527 introduced spacing presets. The preset values can be customized in settings.spacing.spacingScale or settings.spacing.spacingSizes or settings.spacing.customSpacingSize in theme.json.

On the other hand, classic themes that don't have theme.json can opt-in to the spacing feature via custom-spacing or new appearance-tools. However, as far as I know, there is no way to change this preset value without theme.json.

What is your proposed solution?

It would be nice if spacing presets could be opted in at the PHP level, like font size, color, and gradients.
For example:

add_theme_support(
	'editor-spacing-presets',
	array(
		array(
			'size' => 'clamp(16px, 1.5vw, 20px)',
			'slug' => '45',
			'name' => 'Flexi Normal'
		),
		array(
			'size' => 'clamp(1.4rem, 2.15vw, 4rem)',
			'slug' => '55',
			'name' => 'Flexi Large'
		)
	)
);

In theme.json, spacingSizes are available to automatically generate presets, but for theme support, I prefer to simply specify variations in an array.