Add NoDiscard attributes

[Baptouuuu]

Fix #6

[codecov]

Codecov Report

❌ Patch coverage is 97.82609% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.58%. Comparing base (e17aed3) to head (f934095).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/RGBA.php	90.90%	2 Missing ⚠️
src/CMYKA.php	95.83%	1 Missing ⚠️
src/HSLA.php	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop       #9   +/-   ##
==========================================
  Coverage      99.58%   99.58%           
  Complexity       596      596           
==========================================
  Files             34       34           
  Lines           2904     2904           
==========================================
  Hits            2892     2892           
  Misses            12       12           

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