fix: make axislegend's margin themeable

[awadell1]

Description

Fixes #2415

Update axislegend to allow for a themeable margin, falling back to the existing default. Kwarg still takes priority.

I'm suspect that this is not the ideal way to get the theme's Legend.margin entry, but I couldn't find a clear alternative.

Type of change

Delete options that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.