CLI quality of life improvements

[jeremander]

Small quality of life improvements to the command-line interface in main.py:

• Allow -h as abbreviation for ーhelp
• Add version subcommand to display the version of mesa
• Change runserver implementation to use subprocess instead of exec; handle error if run.py doesn't exist

How to Test

• Once installed, run mesa -h on command-line; this should make help menu appear.
• Run mesa version and it should display your installed version of mesa.
• Run mesa runserver within a Mesa project, and it should launch in a browser as usual.
• Run mesa runserver in a folder that isn't a Mesa project; it should error and exit gracefully.

[jackiekazil]

Can you add a how to test section?
And start the commit message with verb? :-)

[codecov]

Codecov Report

Patch coverage: 53.84% and project coverage change: +0.01 🎉

Comparison is base (78ff484) 81.81% compared to head (f159032) 81.83%.

❗ Current head f159032 differs from pull request most recent head ccf5b02. Consider uploading reports for the commit ccf5b02 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1640      +/-   ##
==========================================
+ Coverage   81.81%   81.83%   +0.01%     
==========================================
  Files          18       18              
  Lines        1386     1393       +7     
  Branches      271      272       +1     
==========================================
+ Hits         1134     1140       +6     
- Misses        206      207       +1     
  Partials       46       46              

Impacted Files	Coverage Δ
mesa/main.py	79.41% <53.84%> (+1.63%)	⬆️

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[jackiekazil]

This is lovely. Thank you!