feat: NamespaceIndex alias for uint16_t

[lukasberbuer]

Add NamespaceIndex alias for uint16_t.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 94.65%. Comparing base (41c60cd) to head (ea33ec9).

Files	Patch %	Lines
src/types/Builtin.cpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #219   +/-   ##
=======================================
  Coverage   94.65%   94.65%           
=======================================
  Files         105      105           
  Lines        5405     5405           
  Branches      721      714    -7     
=======================================
  Hits         5116     5116           
  Misses        170      170           
  Partials      119      119           

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