Refactor NpmLocation class

[FrederikBolding]

Refactor NpmLocation to move most logic to a BaseNpmLocation class that can be extended in the mobile client. A class extending the base class only need to implement fetchNpmTarball reducing code duplication. As part of this refactor I've also simplified the implementation in multiple places and improved the coverage.

Fixes #2040

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (58b9318) 96.13% compared to head (7af8763) 96.22%.

Files	Patch %	Lines
...ckages/snaps-controllers/src/snaps/location/npm.ts	97.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2038      +/-   ##
==========================================
+ Coverage   96.13%   96.22%   +0.09%     
==========================================
  Files         270      270              
  Lines        6309     6310       +1     
  Branches     1023     1023              
==========================================
+ Hits         6065     6072       +7     
+ Misses        244      238       -6     

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