Add a figure_format themeable

[has2k1]

This PR adds figure_format theme control for notebook display and default saves.

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.50%. Comparing base (0e8f227) to head (936b057).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
plotnine/composition/_compose.py	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1127      +/-   ##
==========================================
+ Coverage   88.12%   88.50%   +0.37%     
==========================================
  Files         226      226              
  Lines       16579    16613      +34     
  Branches     2121     2130       +9     
==========================================
+ Hits        14611    14703      +92     
+ Misses       1356     1295      -61     
- Partials      612      615       +3     

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