Consider adding "snapping" to resizable template part focus mode

[annezazu]

What problem does this address?

Right now, work is underway to add snapping to specific widths for resizing images #44357 and this work could serve as inspiration for snapping for the template part focus mode experience as currently it's not clear where various rough breakpoints might be. To be clear, this view:

snapping.mov

What is your proposed solution?

Add in a form of snapping for various common viewpoints. My main concern and question mark is if this goes against the pursuit of intrinsic design: #34641

In my mind, you would be able to both set things to a custom width while also getting some visual "snapping" indication when you hit common points as we have now in the post editor.

[github-actions]

Hi,
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