Menu Item placeholder: Show page label in setup state

[jasmussen]

What problem does this address?

This is a sub-task oriented part of #32373. As discussed in that ticket, we need for the Navigation block to be able to accommodate a diverse range of opinionated patterns such as this one:

One part of that could be to indicate missing information through an underline, like so:

That missing information could be:

1. The label
2. The URL
3. Both

What is your proposed solution?

#32512 adds a wavy underline to menu items that have neither a URL nor a label. That checks off item 3 and 1 on the above list.

Item 2 still need addressing — that's the case where the label has already been configured, but the URL is still missing. In the following example markup, the first navigation link is missing both URL and label. The second navigation link is only missing the URL, but has "Services" pre-set as its label.

<!-- wp:navigation {"orientation":"horizontal"} -->
<!-- wp:navigation-link {"type":"page","kind":"post-type"} /-->
<!-- wp:navigation-link {"label":"Services","type":"page","kind":"post-type"} /-->
<!-- /wp:navigation -->

By allowing the label to be pre-set, this would allow pattern authors to create a wide range of highly useful block patterns where all you have to do is click an item and point at the URL:

In the above mockup, the URL dialog is also redesigned to surface both label and URL. This is ticketed separately in #30170,

[jasmussen]

Worth clarifying that this issue is dependant on #32512 landing first. I think the wavy underline is worth trying — it's easy to tweak the thickness and shape of it, and if we ultimately find it not working, the same spot in the CSS can be used to show some other indicator, like a box, a dot, or anything else.

[jasmussen]

Per conversation in #35094 there were concerns both with link behavior and the obviousness of patterns where the label is pre-set, so perhaps this is not the best path forward after all. As an exploration, this took it to the extreme, and now having tried it in a branch, we can pull back a little. I'll update the tracking issue as well.