Add standalone mapscale() function

[joa-quim]

Summary

• Add mapscale() / mapscale!() convenience function that wraps basemap for drawing map scale bars (-L)
• All scale sub-options (anchor, scale_at_lat, length, fancy, label, align, units, vertical, etc.) are top-level kwargs
• Uses the Dict-based dispatch pattern (no kwargs in the main function) to avoid recompilation
• Defaults to frame=:none so only the scale bar is drawn

Test plan

• Standalone with explicit R/J and inside/anchor placement
• Overlay mode (mapscale!) reusing existing plot R/J
• Default frame=:none (no axes drawn)
• Options: units, align, label, fancy, box

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.52%. Comparing base (5be0b89) to head (3740b77).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1918      +/-   ##
==========================================
- Coverage   85.79%   85.52%   -0.27%     
==========================================
  Files         186      186              
  Lines       27510    27157     -353     
==========================================
- Hits        23601    23226     -375     
- Misses       3909     3931      +22     

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