refactor: let router specify its state shape

[satya164]

No description provided.

[codecov-io]

Codecov Report

Merging #26 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #26   +/-   ##
=======================================
  Coverage   89.51%   89.51%           
=======================================
  Files          18       18           
  Lines         248      248           
  Branches       60       60           
=======================================
  Hits          222      222           
  Misses         24       24           
  Partials        2        2

Impacted Files	Coverage Δ
src/useDescriptors.tsx	100% <ø> (ø)	⬆️
src/BaseRouter.tsx	38.46% <ø> (ø)	⬆️
src/useOnAction.tsx	95.65% <ø> (ø)	⬆️
src/BaseActions.tsx	61.53% <ø> (ø)	⬆️
src/useOnRouteFocus.tsx	84.61% <ø> (ø)	⬆️
src/useNavigationBuilder.tsx	100% <100%> (ø)	⬆️
src/NavigationContainer.tsx	100% <100%> (ø)	⬆️
src/__tests__/__fixtures__/MockRouter.tsx	90.47% <100%> (ø)	⬆️

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