Support presets for column block widths

[cbirdsong]

What problem does this address?

Right now there are de facto preset widths for the column block thanks to these variations:

However, a user that picks one of these and changes their mind is faced with a text field they have to manually enter a width into:

This is particularly confusing if they picked one of the options with equal width columns, as the column width field defaults to a pixel width instead of a percentage:

What is your proposed solution?

The column block width field should start as a list of presets that the user picks from, with a "custom" as a secondary option, similar to the way text or spacing size presets work.

This change would also have other benefits. CSS values like "auto", "max-content" and "min-content" that currently can't be used could be made available as presets, making the column block more flexible. Presets would also allow the editor to generate classes for width settings instead of inline styles, improving compatibility with strict CSPs.

Related issues:

• Proposal: Standardized block markup, theme.json design tokens, and CSS classes to improve interoperability #38998
• Adopt a classes-first approach to CSS #54033
• Resetting block controls to their default state is often unintuitive and is sometimes impossible #55258