feat: "gen_completions" command

[shawn111]

I'm submitting a

• bug fix
• feature
• documentation addition

What is the motivation / use case for changing the behavior?

$ comtrya gen-completions --help
Auto generate completions

for examples:
 - bash: ```source <(comtrya gen-completions bash)```
 - fish: ```comtrya gen-completions fish | source```

Usage: comtrya gen-completions <SHELL>

Arguments:
  <SHELL>
          If provided, outputs the completion file for given shell

          [possible values: bash, elvish, fish, powershell, zsh]

Options:
  -h, --help
          Print help (see a summary with '-h')

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 28.36%. Comparing base (b574682) to head (97e94ac).
Report is 19 commits behind head on main.

Files	Patch %	Lines
app/src/commands/gen_completions.rs	0.00%	5 Missing ⚠️
app/src/main.rs	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #422      +/-   ##
==========================================
- Coverage   28.59%   28.36%   -0.24%     
==========================================
  Files          85       86       +1     
  Lines        2301     2313      +12     
==========================================
- Hits          658      656       -2     
- Misses       1643     1657      +14     

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