Add split sharp edges to theme

[akaszynski]

Add in the ability to set splitting of sharp edges in the global theme. This makes it easier to set default edge splitting for plots with sharp edges (like CAD-like geometry).

>>> import pyvista
>>> pyvista.global_theme.sharp_edges_feature_angle = 45
>>> pyvista.global_theme.split_sharp_edges = True
>>> pyvista.global_theme.split_sharp_edges
True

Bonus (there always is)

• Add a fixture to reset the global theme so it's not modified during docstring testing.
• Remove doctest:+SKIP within theme.py as it is no longer necessary due to proper theme resetting.

[codecov]

Codecov Report

Merging #2575 (93e1b46) into main (a0979f7) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2575      +/-   ##
==========================================
+ Coverage   93.69%   93.75%   +0.06%     
==========================================
  Files          75       75              
  Lines       16159    16180      +21     
==========================================
+ Hits        15140    15170      +30     
+ Misses       1019     1010       -9     

[adeak]

Great idea adding the fixture, lot more doctests are always awesome. Two minor suggestions but this is good as is too. Thanks.