Nav Block - create from existing Menus

[getdave]

In the original Issue around the initial state for the placeholder for the Nav Block we discussed having the ability to create a Nav from an existing WP Menu.

We can't do this because of the lack of a REST API endpoint. There is also an Issue for that.

Once that Issue is resolved we should look to add this functionality to the Block.

Describe the solution you'd like
Ability to select from an existing WP Menu and have a Nav created from that.

[getdave]

Highly relevant comment on getting the API endpoint launched

#18373 (comment)

[retrofox]

I've been working as well in an approach to this issue. Let me paste the very draft PR

[getdave]

I'm going to take this up again now.

[getdave]

@shaunandrews Pinging you because you were involved in the original implementation. If you've had any further thoughts on the design implementation of this please do let us know. We're working on the technical implementation now so would be good to have the design to implement alongside it also. Much appreciated.

cc @karmatosed.

[shaunandrews]

Here's a quick mockup of how we could adapt the initial setup state to allow for choosing existing menus:

[karmatosed]

I think this simple adaption of the 'create empty' to be a drop down works @shaunandrews. My only ponder is about making 'new' say 'create' and maybe be at the top.

[paaljoachim]

How is this feature coming along?
If the Nav Block is shipped with WordPress 5.4. This will be one missing feature that I believe will often come up in the support forum as well as other places. Answering with that one can not add an pre-existing menu at this time to the Nav block does not look good. If it is possible to add to the feature while in beta then that is great. Throwing a likely solution quickly together before sometime Monday so one can use a month during beta to improve it could also be a solution (not a great solution but one solution to get it in).

@getdave

[spacedmonkey]

I have created a PR for the menus endpoints - #20292 based on the feature plugin https://github.com/WP-API/menus-endpoints