change: default config location on macOS considers $XDG_CONFIG_HOME

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (such as any change that modifies the UI), please provide screenshots of the changes:

Actually support $XDG_CONFIG_HOME on macOS. Apparently in our docs we also say we do, but we, uh, don't, because dirs doesn't.

Note this is backwards-compatible, in that if a config file exists in the old default locations, we will check those first.

Issue

If applicable, what issue does this address?

Closes: #1569

Testing

If relevant, please state how this was tested. All changes must be tested to work:

If this is a code change, please also indicate which platforms were tested:

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 41.65%. Comparing base (74ae124) to head (6712e6d).
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/options.rs	86.66%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1570      +/-   ##
==========================================
+ Coverage   41.46%   41.65%   +0.19%     
==========================================
  Files         114      114              
  Lines       17840    17893      +53     
==========================================
+ Hits         7398     7454      +56     
+ Misses      10442    10439       -3     

Flag	Coverage Δ
macos-12	36.94% <86.66%> (+0.21%)	⬆️
ubuntu-latest	43.38% <0.00%> (-0.06%)	⬇️
windows-2019	36.87% <0.00%> (-0.06%)	⬇️

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