Project environment variable management commands

[akx]

Rehashes @drazendee's PR from 2022, now with rebasing and tests!

Closes #251 (supersedes it)

[codecov]

Codecov Report

Attention: Patch coverage is 97.24771% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.54%. Comparing base (d6ffdfb) to head (8495f8e).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...i/commands/project/environment_variables/delete.py	90.00%	1 Missing and 1 partial ⚠️
...cli/commands/project/environment_variables/list.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
+ Coverage   86.30%   86.54%   +0.24%     
==========================================
  Files         133      138       +5     
  Lines        4716     4824     +108     
  Branches      605      616      +11     
==========================================
+ Hits         4070     4175     +105     
- Misses        452      454       +2     
- Partials      194      195       +1     

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[teroyks]

LGTM!