Allow user to change breakpoint for mobile menu

[Nyiriland]

Issue

Currently, the default breakpoint for the mobile menu is under 600px. This means that desktop header navs with long menus generally wrap in a very unattractive way before the breakpoint kicks with a hamburger button. See exaggerated example:

This is one thing I almost always end up overriding with the CSS customizer.

Suggestion

It would be great to allow the user to add their own breakpoint for the mobile menu. I think it would make sense to add this to the Navigation block, under the Display section, if the "Mobile" option is chosen. Quick mockup below.

[kathrynwp]

related: #42305

[Azragh]

#13363
#20244
#35808
#35848
#40870

[helixgdn]

I posted a solution here:

#54388 (comment)