Improve Notebook 7 functionality and testing

[bollwyvl]

Checklist

• [x] ran doit lint locally

References

• fixes #56
• blocked by https://github.com/jupyterlite/jupyterlite/pull/1249

Code changes

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

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 :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/deathbeds/jupyterlab-deck/pull/59/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deathbeds) | Coverage Δ | | |---|---|---| | [front-end](https://app.codecov.io/gh/deathbeds/jupyterlab-deck/pull/59/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deathbeds) | `93.09% <94.91%> (-0.63%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deathbeds#carryforward-flags-in-the-pull-request-comment) to find out more.

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