Add XDG_PROJECTS_DIR support

[shelepuginivan]

The recent release 0.20 of xdg-user-dirs introduced XDG_PROJECTS_DIR user directory. The original proposal describes it as a directory for user software projects, like VCS directories, compiled binaries, etc. This PR adds support for this directory.

The default paths are similar to other user directories, such as XDG_DOCUMENTS_DIR. However, I couldn't find a Windows known folder with similar purpose. There are folders for installed programs, but from what I can tell there is no standard for "software projects". For now the Windows implementation just checks the environment variable and uses %USERPROFILE%\Projects as a fallback.

Let me know what you think.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fd7dadd) to head (cfa09e3).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #148   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            9         9           
  Lines          418       427    +9     
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+ Hits           418       427    +9     

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[adrg]

Hi @shelepuginivan,

Thank you for the PR. Looks good.
I also checked the known folder list and could not find any suitable equivalent either. So not providing one is fine.