Mark Navigation as beta experimental

Yay! We're in core! Now let's get in 10.3.0!

I noticed that there are some comments missing, and other minor things that could be improved.

So, I don't see any beta blockers listed in #3421969: [PLAN] New Navigation and Top Bar to replace Toolbar Roadmap: Path to Stable. Is this the only blocker? If so, can you elaborate or should we go though again?

What actionable steps are needed to get this into 10.3.0?

From a Framework manager POV I'm happy to mark navigation as beta level stability, there's no changes outside the module, all the code is final and internal and the data model reuses the existing layout builder schema

I've reviewed both the initial MR (a bit) and the roadmap issue/sub-issues (a fair bit), and I think @longwave has also reviewed the roadmap. There is a fair bit to do but it's all beta-safe, and we're already seeing more testing after the commit, so happy removing the RM tag here.

I also have no objections to it being marked beta. (Don't credit me though; did not put time into reviewing it myself).

Aside: This is the issue to tag for both release notes and release highlights, if/when the FEFM signoff is given.

Going through the JS it all looks fine for the Navigation module but I'm a little unsure about it becoming an API available to other extensions. Instead of dealing with the buzzkill of last minute JS change requests, we can address my concerns about this being marked beta experimental by just making the libraries internal #3444932: Make libraries with JS internal, at least for beta

Same here, the JS is ok with me for a Beta, for stable i want to use proper events like we did for dialog but that can wait (since we'll be marking all the JS apis internal for now), there is generally too much nesting, some things can be refactored but nothing impacting the user experience, it's mostly about DX and maintenance. Better get this feature in users' hands first.

Adding tag per #9