Replace focus trap with `inert`

[OliverJAsh]

Now that browsers have good support for inert, could we consider replacing the custom focus trap logic with this?

Alternatively, we could use the dialog element so we get this behaviour for free.

[diasbruno]

I'll be more conservative with this changes that relies on the browser.
I mean, people that uses the browser are ok, but projects that links with different versions of webviews can be affected.
I'm in favor of delegating the trap logic to a library. The problem is: which library?