Refactors and typescript improvements for jbrowse-web loader

[cmdcolin]

I was investigating this issue #3996 and found a possible solution, but in the process made some miscellaneous typescript and code organization refactors

[codecov]

Codecov Report

Merging #4005 (245cb9c) into main (83bdd1c) will increase coverage by 0.01%.
Report is 2 commits behind head on main.
The diff coverage is 71.42%.

❗ Current head 245cb9c differs from pull request most recent head 6a72040. Consider uploading reports for the commit 6a72040 to get more accurate results

@@            Coverage Diff             @@
##             main    #4005      +/-   ##
==========================================
+ Coverage   63.65%   63.66%   +0.01%     
==========================================
  Files        1023     1023              
  Lines       30205    30220      +15     
  Branches     7200     7207       +7     
==========================================
+ Hits        19226    19241      +15     
  Misses      10814    10814              
  Partials      165      165              

Files	Coverage Δ
packages/core/PluginLoader.ts	47.05% <100.00%> (+4.90%)	⬆️
products/jbrowse-web/src/SessionLoader.ts	70.68% <100.00%> (ø)
products/jbrowse-web/src/createPluginManager.ts	62.16% <ø> (ø)
products/jbrowse-web/src/util.ts	68.36% <ø> (ø)
...browse-web/src/components/SessionWarningDialog.tsx	41.17% <60.00%> (+5.46%)	⬆️
...jbrowse-web/src/components/ConfigWarningDialog.tsx	0.00% <0.00%> (ø)
products/jbrowse-web/src/components/Loader.tsx	88.88% <82.14%> (+1.85%)	⬆️

... and 3 files with indirect coverage changes

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