Extract function to create custom gitignore file

[fabiofortkamp]

This attemps to solve #403 by including a keyword argument to the initialize_project function to customize the folders to be included in the .gitignore file. Default options is to preserve the current gitignore file.

[Datseris]

Thanks, this is an example of a perfect pr: has tests, docs, and increments project version. will merge once tests pass!

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (cad2a38) 90.65% compared to head (962c02f) 90.47%.

Files	Patch %	Lines
src/project_setup.jl	93.93%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #404      +/-   ##
==========================================
- Coverage   90.65%   90.47%   -0.19%     
==========================================
  Files           8        8              
  Lines         760      787      +27     
==========================================
+ Hits          689      712      +23     
- Misses         71       75       +4     

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