Add infrastructure for visual tests and first test(s)

[astrofrog]

This adds infrastructure to do visual tests using Solara

I've also made use of pytest-mpl which gives the following benefits:

• Nice gallery generated for each PR showing any differences in reference images
• Ability to store only the hashes in this repo and have reference images be automatically pushed to glue-jupyter-visual-tests

Note that we use CircleCI as in astropy because it allows the gallery to be hosted/rendered directly.

At the moment this requires a small hack which is to show the generated image in a Matplotlib figure but hopefully we can simplify things as mentioned in matplotlib/pytest-mpl#201

For now I also need to use a fork of Solara with widgetti/solara#167

Gallery preview: https://output.circle-artifacts.com/output/job/6d90ef24-a6a1-49e4-a7d0-fc5ac19e4b36/artifacts/0/results/fig_comparison.html

[codecov]

Codecov Report

Patch coverage: 47.61% and project coverage change: -0.57 ⚠️

Comparison is base (9d44ed5) 89.58% compared to head (286f513) 89.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #360      +/-   ##
==========================================
- Coverage   89.58%   89.02%   -0.57%     
==========================================
  Files          86       88       +2     
  Lines        4697     4757      +60     
==========================================
+ Hits         4208     4235      +27     
- Misses        489      522      +33     

Impacted Files	Coverage Δ
glue_jupyter/bqplot/scatter/tests/test_visual.py	27.27% <27.27%> (ø)
glue_jupyter/tests/helpers.py	55.26% <55.26%> (ø)
glue_jupyter/app.py	87.09% <100.00%> (ø)

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