Introduce experiment for inspector based navigation editing

[draganescu]

What?

This PR a spin off from #44534.
The base PR was approved but it was too big as a changeset so we're splitting it up.

This PR introduces the experiment setting and checkbox required to build as an experiment a new inspector based editing UX
for the navigation block - based on a list view.

Why?

To develop this new UX in isolation, potetially faster, and enable user validation of the UX.

How?

Copy paste of the zoomed out view setting and checkbox and then change the copy.

Testing Instructions

1. Go to Gutenberg > Experiments
2. There should be a new experiment named "Test a new "off canvas" editor for navigation block using the block inspector
   and a tree view of the current menu"

Screenshots or scree ncast

[codesandbox]

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[scruffian]

LGTM