Implement expand extension interface and Cell struct

[MichaelHatherly]

Now that the worker code is an actual package, we can provide a way for 3rd-party packages to hook into the behaviour of the worker without us having to host all the integrations within this code base. E.g.

[weakdeps]
QuartoNotebookWorker = "38328d9c-a911-4051-bc06-3f7f556ffeda"

[extensions]
MyPackageNameQuartoNotebookWorkerExt = "QuartoNotebookWorker"

This adds expand and Cell that a package can use to hook in to the notebook cell expansion feature.

[codecov]

Codecov Report

Attention: Patch coverage is 21.62162% with 29 lines in your changes are missing coverage. Please review.

Project coverage is 53.26%. Comparing base (63c8a52) to head (315c8bf).

Files	Patch %	Lines
src/QuartoNotebookWorker/src/render.jl	0.00%	16 Missing ⚠️
src/QuartoNotebookWorker/src/cell_expansion.jl	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #135      +/-   ##
==========================================
- Coverage   53.62%   53.26%   -0.37%     
==========================================
  Files          26       27       +1     
  Lines        1199     1209      +10     
==========================================
+ Hits          643      644       +1     
- Misses        556      565       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.