Navigation: Automatically name menus and remove "Save as" button from toolbar

[mkaz]

The navigation block saves its content to a custom post type, this helps persist it across classic and block theme switches. This mechanism to save should not impede the user flow when creating a menu.

This was discussed in a different PR, pulling out @mtias comment from #36169 (comment):

It's important saving menus is as transparent to users as we can conjure.
Untitled menus is a poor experience; a user should not have to name things.
We should aim to auto-generate a name based on the template part a navigation is used. If a menu is added to a template root, we should use that, or just a numbered one.

Example: navigation-header, navigation-header-with-banner, navigation-1, navigation-archive, etc.

Menus can be renamed in the inspector, not in the toolbar, and the "Save as" button can then be removed.

cc: @jasmussen @adamziel @getdave - I know y'all might be creating similar must-have tickets, I didn't see another matching this one, so wanted to make sure it was captured.

[mkaz]

Is this a duplicate of #36524? It looks like the proposed solution in that issue would solve this, we can close this since there is more discussion and background there.

[talldan]

@mkaz #36524 is about changing the menu id to be a slug, and automatically generating that slug based on where the block is rendered (I imagine from the template part area that the block is within).

At the moment, the menu name is a separate thing (a post title). It's unclear to me whether the idea is to remove the menu name and only have a slug. Maybe that's the case, if it is this can be closed as a duplicate (cc @mtias - would you be able to clarify?).

[mtias]

Name and slug should match, we should generate a name "{Template Part Name} Menu" that WP also saves as a slug as it normally does. The name is useful for the focused template part display, etc.

Regarding the "save as" the idea is to align with other blocks that have saving flows (reusable blocks, etc). This last point is not targeted for 5.9 as it's an ongoing area of work and ideation.

[mkaz]

Ok, I'm going to close this one and reopen @tellthemachines ticket #36593 that is specific to the "Save as" part, there more details and a screenshot in that ticket so better to work off.

So then ticket #36524 can cover both name and slug per Matias comment above.