Add NoDiscard attribute on mutation free methods

[Baptouuuu]

Fix #5

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.36%. Comparing base (e205ae8) to head (9fa2fa5).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/Environment.php	71.42%	2 Missing ⚠️
src/Application.php	93.33%	1 Missing ⚠️
src/Http/Route.php	90.90%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop      #17      +/-   ##
=============================================
+ Coverage      93.71%   95.36%   +1.65%     
  Complexity       123      123              
=============================================
  Files             14       14              
  Lines           1447     1447              
=============================================
+ Hits            1356     1380      +24     
+ Misses            91       67      -24     

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