Allow using non-SVG icons in toolbar

[Carifio24]

Currently the way the Jupyter toolbar is set up, we can only use SVG icons. This is usually fine, as most of our glue-core icons are available as both png or svg. However, there are some icons (the rotation icon in the vispy viewers, for example) for which we only have pngs.

This PR updates the format that gets passed to icon_path to depend on the path for the icon. If a valid image MIME type can't be determined, then it raises an error.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.06%. Comparing base (1d9c74e) to head (93005e7).
Report is 16 commits behind head on main.

Files	Patch %	Lines
glue_jupyter/common/toolbar_vuetify.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #465      +/-   ##
==========================================
+ Coverage   86.04%   86.06%   +0.02%     
==========================================
  Files          90       90              
  Lines        5224     5240      +16     
==========================================
+ Hits         4495     4510      +15     
- Misses        729      730       +1     

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