[Feature] Add env_is_... helper functions

[titouanmathis]

Add the following helper functions and methods for easier environment testing:

use Studiometa\WPToolkit\Helpers\Env;
use function \Studiometa\WPToolkit\env_is_prod;
use function \Studiometa\WPToolkit\env_is_preprod;
use function \Studiometa\WPToolkit\env_is_local;
use function \Studiometa\WPToolkit\env_is_staging;
use function \Studiometa\WPToolkit\env_is_development;

env_is_prod(); // bool
env_is_preprod(); // bool
env_is_local(); // bool
env_is_staging(); // bool
env_is_development(); // bool

Env::is_prod(); // bool
Env::is_preprod(); // bool
Env::is_local(); // bool
Env::is_staging(); // bool
Env::is_development(); // bool

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.98%. Comparing base (5bdc7e3) to head (46d8348).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop      #31      +/-   ##
=============================================
+ Coverage      62.81%   63.98%   +1.17%     
- Complexity       194      201       +7     
=============================================
  Files             14       14              
  Lines            554      572      +18     
=============================================
+ Hits             348      366      +18     
  Misses           206      206              

Flag	Coverage Δ
unittests	63.98% <100.00%> (+1.17%)	⬆️

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