[DevTools] Rename NativeElement to HostInstance in the Bridge (#30491)

Stacked on #30490.

This is in the same spirit but to clarify the difference between what is
React Native vs part of any generic Host. We used to use "Native" to
mean three different concepts. Now "Native" just means React Native.

E.g. from the frontend's perspective the Host can be
Highlighted/Inspected. However, that in turn can then be implemented as
either direct DOM manipulation or commands to React Native. So frontend
-> backend is "Host" but backend -> React Native is "Native" while
backend -> DOM is "Web".

Rename NativeElementsPanel to BuiltinElementsPanel. This isn't a React
Native panel but one part of the surrounding DevTools. We refer to Host
more as the thing running React itself. I.e. where the backend lives.
The runtime you're inspecting. The DevTools itself needs a third term.
So I went with "Builtin".