fix(no-unused-variables): Stop reporting reassigned function and mixin parameters as unused

[MorevM]

Closes #58

Parameters reassigned inside conditional at-rules are now recognized as used
when referenced later in the function or mixin.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.64%. Comparing base (864a93b) to head (a030d53).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #64   +/-   ##
=======================================
  Coverage   98.63%   98.64%           
=======================================
  Files          56       56           
  Lines        1322     1329    +7     
  Branches      403      405    +2     
=======================================
+ Hits         1304     1311    +7     
  Misses         16       16           
  Partials        2        2           

Files with missing lines	Coverage Δ
...es/sass/no-unused-variables/no-unused-variables.ts	98.57% <100.00%> (+0.15%)	⬆️

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