Use a user-supplied fetchCJS callback to import CJS plugins

[cmdcolin]

This PR is similar to #2891 but for jbrowse-desktop, which can import CJS modules

Without this PR, a 'http' module polyfill (stream-http) is bundled in jbrowse-web

screenshots with npx source-map-explorer build/static/js/*.js in jbrowse-web

before (see stream-http)

after (stream-http removed)

Also uses window.require instead of __non_webpack_require__ which sort of ensures that the electron require machinery is used instead of bundled

[codecov]

Codecov Report

Merging #2904 (4e6eb22) into main (8c0c603) will increase coverage by 0.03%.
The diff coverage is 3.03%.

@@            Coverage Diff             @@
##             main    #2904      +/-   ##
==========================================
+ Coverage   59.90%   59.93%   +0.03%     
==========================================
  Files         587      587              
  Lines       26630    26633       +3     
  Branches     6472     6473       +1     
==========================================
+ Hits        15952    15962      +10     
+ Misses      10376    10369       -7     
  Partials      302      302              

Impacted Files	Coverage Δ
products/jbrowse-desktop/src/StartScreen/util.tsx	0.00% <0.00%> (ø)
packages/core/PluginLoader.ts	24.17% <14.28%> (+5.75%)	⬆️
packages/core/util/layouts/GranularRectLayout.ts	87.17% <0.00%> (+0.42%)	⬆️
...inearGenomeView/components/RefNameAutocomplete.tsx	80.80% <0.00%> (+3.03%)	⬆️
...lignments/src/BamAdapter/BamSlightlyLazyFeature.ts	84.37% <0.00%> (+3.12%)	⬆️
packages/core/TextSearch/TextSearchManager.ts	100.00% <0.00%> (+3.44%)	⬆️
...src/SvgFeatureRenderer/components/FeatureLabel.tsx	86.20% <0.00%> (+6.89%)	⬆️

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[cmdcolin]

tested with the cjs bundle of jbrowse-plugin-systeminformation example from https://github.com/garrettjstevens/jbrowse-plugin-systeminformation and worked :)