Data Views - Slug Field Control: Implement validation

[gigitux]

The Slug Field Control currently lacks validation, allowing the save button to be clickable even when the input is empty:

This isn’t ideal because a page/post should always have a slug set. Currently, if the input is empty, the original slug is restored on the onBlur event, which is not the best UX.

Technical Investigation Required

There is an edge-case that I think that we should take care, but I will wait for your feedback before to proceed. When the input is empty, the save button is clickable:
It's ok to address this in a follow-up, but wanted to share some thoughts:

• we already validation in fields (isValid)
• it's already built-in in the types (if your field declares a type it doesn't have to have its own isValid)
• there's an utility to validate all fields based on their isValid callback: isItemValid that we are already using in some scenarios >(reorder page modal)

Thinking about how to scope next steps, this is what I think we should do:

• Connecting the existing isItemValid utility to the save button in the site editor.
• Implement isValid in the slug field introduced here to check for non empty.

Note that when we introduce "slug" field type, that should be automatically created for the field — but that requires other things we don't have yet (being able to register new fields types).

Originally posted by @oandregal in #65196 (comment)