…ptions are passed by consumers

- Previously useState() was caching initial styles/row heights so that developer prop changes (if not overridden by user settings) weren't reflected in the UI

- This fix:
  - Adds `useEffect`s that fires whenever the merged consumer/user states change (so will automatically always set UI state based on most recent configurations)
   - Refactors out the need for the `intial*` vars
  - Refactors out the need for manual UI state setting within UI callbacks
  - Cleans up resetToInitialState callback to only have to set user configs back to empty objs

- now that we're no longer storing initial UI state

- NB: this new `showResetButton` state is slightly less granular than the previous logic, because it doesn't check if the display state was changed back to the original initial state and only checks for a coarse 'did the user interact with the controls', but IMO this is an acceptable tradeoff and adds minimal friction in exchange for easier developer maintainability

- our merged configs are guaranteed to always be at least an empty object, so we can now remove some undefined checks

- unfortunately `shallow` in Enzyme does not invoke useEffect, so the lineCount UI no longer appears in the changed test. I modified the test to simply no longer have that call (as it was tangential to the main test case)