fix(parsetorgb): fix rgba/hsla alpha precision

[levymetal]

• Fix https://github.com/styled-components/polished/issues/610
• Add more test cases for invalid rgba/hsla strings
• Update docs

[codecov[bot]]

Codecov Report

Merging #611 (880f5d2) into main (bee4d68) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #611   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           89        89           
  Lines          860       860           
  Branches       323       323           
=========================================
  Hits           860       860           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/color/parseToRgb.js	100.00% <100.00%> (ø)

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