swidth: use cross platform implementation, fixes #7493

[ThomasWaldmann]

This implementation should be good enough for our usecase (paths) and has no external dependencies.

There is also a wcwidth library which might be even better, but would add another dependency.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.89%. Comparing base (50f4e54) to head (37fbe3f).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/borg/platform/base.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9283      +/-   ##
==========================================
+ Coverage   75.86%   75.89%   +0.03%     
==========================================
  Files          86       86              
  Lines       14758    14761       +3     
  Branches     2195     2198       +3     
==========================================
+ Hits        11196    11203       +7     
+ Misses       2889     2884       -5     
- Partials      673      674       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.