Improve Notebook 7 functionality and testing

[bollwyvl]

Checklist

• ran doit lint locally

References

• fixes Expand Notebook 7 acceptance tests #56
• blocked by Add markdownviewer-extension to edit app jupyterlite/jupyterlite#1249

Code changes

• improve handling of markdown preview
  • open preview as deck if a markdown editor is opened
  • fix live updating CSS jank, hide menus, etc.
• update acceptance tests
  • markdown preview

User-facing changes

• markdown documents will get a deck toolbar button
• starting a deck will start the preview
• the "sidebar" can then be expanded to hide the source

before	default split	wide

Backwards-incompatible changes

• ??

Future Work

• sync scrolling???
• determine if some DOM-based pattern can be used for subslides, e.g.

  <!-- maybe a future front-matter slide -->
  ---
  # The First Thing
  
  ---
  ---
  
  ## A sub-thing
  
  ---
  ---
  
  <!-- this sub-thing intentionally left blank -->
  
  ---
  
  # The Next Thing
  
  

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (cb67d32) 93.75% compared to head (b5edad2) 93.14%.

Files	Patch %	Lines
js/jupyterlab-deck/src/markdown/presenter.ts	92.68%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
- Coverage   93.75%   93.14%   -0.62%     
==========================================
  Files          15       16       +1     
  Lines        1282     1327      +45     
  Branches      264      274      +10     
==========================================
+ Hits         1202     1236      +34     
- Misses         39       46       +7     
- Partials       41       45       +4     

Flag	Coverage Δ
front-end	93.09% <94.91%> (-0.63%)	⬇️

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