feat!: Allow plural directory and only png's for manifest icons

[dvlden]

This PR resolves #235

• allow only png's
• improve regex to allow plural directory name

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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (207b750) 63.94% compared to head (61e8a03) 63.96%.

Files	Patch %	Lines
src/core/utils/manifest.ts	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   63.94%   63.96%   +0.01%     
==========================================
  Files          75       75              
  Lines        5902     5902              
  Branches      473      474       +1     
==========================================
+ Hits         3774     3775       +1     
+ Misses       2091     2090       -1     
  Partials       37       37              

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