Add extendSession extension point to web and desktop

[garrettjstevens]

This adds an extension point to web and desktop that allows the session to be extended. You can use this to add more—or override exisiting—state, views, or actions to the session. I can be used from plugins like this:

pluginManager.addToExtensionPoint(
  'Core-extendSession',
  (sessionModel: IAnyModelType) =>
    sessionModel
      .props({
        // additional model state here
      })
      .views((self) => ({
        // additional views here
      }))
      .actions((self) => ({
        // additional actions here
      })),
)

[codecov]

Codecov Report

Merging #3179 (d6356e5) into main (a7e7da9) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3179      +/-   ##
==========================================
+ Coverage   59.47%   59.49%   +0.02%     
==========================================
  Files         671      671              
  Lines       28635    28637       +2     
  Branches     6928     6928              
==========================================
+ Hits        17030    17039       +9     
+ Misses      11283    11276       -7     
  Partials      322      322              

Impacted Files	Coverage Δ
...roducts/jbrowse-desktop/src/sessionModelFactory.ts	6.59% <100.00%> (+0.34%)	⬆️
products/jbrowse-web/src/sessionModelFactory.ts	59.45% <100.00%> (+0.12%)	⬆️
...ments/src/SNPCoverageAdapter/SNPCoverageAdapter.ts	58.00% <0.00%> (+0.66%)	⬆️
plugins/alignments/src/BamAdapter/BamAdapter.ts	72.97% <0.00%> (+0.90%)	⬆️
packages/core/ui/SnackbarModel.ts	90.47% <0.00%> (+23.80%)	⬆️

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[cmdcolin]

I think this should be fine! might want to add it to the docs for extension points