Improve primary shell heuristics

[MatthijsBurgh]

Fixes #43

[MatthijsBurgh]

@cottsay I went for the psutil solution. Please review

[MatthijsBurgh]

@cottsay please review

[cottsay]

please review

Has this been tested yet?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (8f1fb0e) to head (892221b).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
colcon_powershell/shell/powershell.py	0.00%	8 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master     #44   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files           2       2           
  Lines          83      90    +7     
======================================
- Misses         83      90    +7     

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[MatthijsBurgh]

Yes, I have tested the logic in bash and in a powershell.