`app_dirs`: Add tests, allow configuration, template replacement

[tony]

AppDirs should allow:

• Custom replacement of variables
  • XDG dir variables
  • Env variables
  • General custom overrides
• Should live in _internals/app_dirs.py
• Should have tests
• Should have documentation
• Should have doctests

[codecov[bot]]

Codecov Report

Merging #287 (f61fdff) into master (e32faea) will increase coverage by 1.29%. The diff coverage is 93.10%.

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
+ Coverage   69.41%   70.70%   +1.29%     
==========================================
  Files          10       12       +2     
  Lines        1007     1065      +58     
  Branches      182      196      +14     
==========================================
+ Hits          699      753      +54     
- Misses        283      284       +1     
- Partials       25       28       +3     

Impacted Files	Coverage Δ
tests/_internal/test_app_dirs.py	90.00% <90.00%> (ø)
src/unihan_etl/_internal/app_dirs.py	96.42% <96.42%> (ø)

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more