feat(Provider): add overwrite prop

[layershifter]

♻️ overwrite prop

This PR adds overwrite prop for Provider component. It allows to reset current context and avoid merging with existing themes. Existing behavior is preserved.

overwrite=false

<Provider theme={themes.teams}> // Teams theme
  <Provider theme={themes.base} /> // Teams + Base theme

overwrite=true

<Provider theme={themes.teams}> // Teams theme
  <Provider overwrite theme={themes.base} /> // Base theme

[layershifter]

ThemeProvider comes from Fela, we should always use overwrite as Fela has own shallow merging.

https://github.com/robinweser/fela/blob/v10.6.1/packages/fela-bindings/src/ThemeProviderFactory.js

[lucivpav]

Let's test this change.

[codecov]

Codecov Report

Merging #1780 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1780      +/-   ##
==========================================
+ Coverage   69.76%   69.77%   +<.01%     
==========================================
  Files         867      867              
  Lines        7353     7354       +1     
  Branches     2158     2136      -22     
==========================================
+ Hits         5130     5131       +1     
  Misses       2215     2215              
  Partials        8        8

Impacted Files	Coverage Δ
...ackages/react/src/components/Provider/Provider.tsx	95.45% <100%> (+0.1%)	⬆️

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