fix: use em units for internal sizing to allow font-size inheritance

[martijnrusschen]

Converts internal sizing units from rem to em in the datepicker stylesheets. This allows users to easily inherit font size from parent elements by overriding the root .react-datepicker font-size to 'inherit' or '1em'.

The default appearance remains unchanged since the root element still uses font-size: 0.8rem.

Fixes #624

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.22%. Comparing base (f11433c) to head (4411352).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6076   +/-   ##
=======================================
  Coverage   99.22%   99.22%           
=======================================
  Files          30       30           
  Lines        3612     3612           
  Branches     1547     1551    +4     
=======================================
  Hits         3584     3584           
  Misses         27       27           
  Partials        1        1           

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