move static methods out of class

[christianrauch]

With the command line parsing not depending on a class member, we can move these functions as static functions out of the 'MesonBuildTask' class.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 20.71%. Comparing base (194e14a) to head (bb463ed).

Files with missing lines	Patch %	Lines
colcon_meson/build.py	0.00%	29 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #23   +/-   ##
=======================================
  Coverage   20.71%   20.71%           
=======================================
  Files           2        2           
  Lines         251      251           
  Branches       51       51           
=======================================
  Hits           52       52           
  Misses        192      192           
  Partials        7        7           

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