refactor(breaking): rename container to UNSAFE_root; introduce root host element

[mdjastrzebski]

Summary

This PR contains breaking changes behind useBreakingChanges feature flag.

Resolves #1296

1. Rename container to UNSAFE_root
2. Display deprecation messages for container (behind useBreakingChanges )
3. Expose root API that returns root host element.

Test plan

All tests pass.

[thymikee]

Is this type accurate? Isn't there a narrower type for host elements?

[mdjastrzebski]

unfortunately no, though we might think about something like that in the future

[pierrezimmermannbam]

I feel like it could be interesting to give more indications on what we think are good use cases for this api (e.g. asserting an element is not rendered) because I don't think it's very intuitive how to use this

[codecov]

Codecov Report

Base: 96.07% // Head: 96.09% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (8597706) compared to base (1e8b3ba).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1298      +/-   ##
==========================================
+ Coverage   96.07%   96.09%   +0.02%     
==========================================
  Files          49       49              
  Lines        3308     3333      +25     
  Branches      497      503       +6     
==========================================
+ Hits         3178     3203      +25     
  Misses        130      130              

Impacted Files	Coverage Δ
src/render.tsx	100.00% <100.00%> (ø)
src/screen.ts	100.00% <100.00%> (ø)

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