Add NoDiscard attribute on mutation free methods and methods returning monads

[Baptouuuu]

Fix #17

[codecov]

Codecov Report

❌ Patch coverage is 92.42424% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.01%. Comparing base (696e993) to head (79241f5).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/Config.php	82.75%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop      #20   +/-   ##
==========================================
  Coverage      90.01%   90.01%           
  Complexity       130      130           
==========================================
  Files             21       21           
  Lines           1062     1062           
==========================================
  Hits             956      956           
  Misses           106      106           

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