Create extension point example

[elliothershberg]

This is something I worked on with @rbuels today. It is one possible implementation for #2032 .

This example relates to the problems I was having with data in the widget model not persisting in #2080, but it turns out that might not be necessary.

However, this type of system could be used in other areas for that PR to remove some of the code updating the LGV plugin.

cc @garrettjstevens

[codecov]

Codecov Report

Merging #2124 (8a0912b) into main (b223419) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2124   +/-   ##
=======================================
  Coverage   62.02%   62.03%           
=======================================
  Files         476      476           
  Lines       22789    22808   +19     
  Branches     5350     5353    +3     
=======================================
+ Hits        14134    14148   +14     
- Misses       8374     8379    +5     
  Partials      281      281           

Impacted Files	Coverage Δ
packages/core/PluginManager.ts	94.49% <100.00%> (+0.34%)	⬆️
plugins/linear-genome-view/src/index.ts	92.59% <100.00%> (+1.28%)	⬆️
products/jbrowse-web/src/sessionModelFactory.ts	59.20% <100.00%> (+0.12%)	⬆️
...nts/src/SNPCoverageRenderer/SNPCoverageRenderer.ts	80.24% <0.00%> (-7.41%)	⬇️
packages/core/util/index.ts	79.89% <0.00%> (ø)
packages/core/util/analytics.ts	93.02% <0.00%> (+2.32%)	⬆️

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[cmdcolin]

interesting stuff...this still in some sense puts a burden on consumers (e.g. jbrowse-web, jbrowse-desktop, embedded) to properly evaluate extension points, and because they are very much stringly/dynamically defined, it is hard to know whether you would evaluate the right ones

[cmdcolin]

since we do have expectations about which things should be evaluated where, it seems like we could make more hardcoded assumptions