Include SCSS files in exports

[saltire]

Description

This PR exposes the source SCSS files via the package's exports so projects can import them.

Problem
This project's SCSS has a lot of useful variables (e.g. $datepicker__selected-color), but no obvious way to change their default values. This way, a project can import the SCSS files with overrides for the variables, instead of setting a bunch of individual attributes. For example:

@use 'react-datepicker/src/stylesheets/variables.scss' with (
  $datepicker__selected-color: #000000,
  $datepicker__font-family: inherit,
);
@use 'react-datepicker/src/stylesheets/datepicker.scss';

If there is a better way of doing this, please let me know!

Changes
Simply add a line to exports in package.json with a wildcard for all SCSS files.

Contribution checklist

• I have followed the contributing guidelines.
• I have added sufficient test coverage for my changes.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.01%. Comparing base (0a980b3) to head (b69ac4c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5560   +/-   ##
=======================================
  Coverage   97.01%   97.01%           
=======================================
  Files          30       30           
  Lines        3350     3350           
  Branches     1398     1416   +18     
=======================================
  Hits         3250     3250           
+ Misses        100       98    -2     
- Partials        0        2    +2     

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