Remove node-canvas from @jbrowse/core dependencies

[cmdcolin]

If we detect isNode is true, then we use global variables nodeCreateCanvas and nodeImage to perform canvas operations. These are global variables that apps that know that they run under node can set (namely, @jbrowse/img which runs under node and our integration tests which perform canvas snapshots)

We do not try to use the global variables until client code calls new Image or createCanvas, which allows us an opportunity to set nodeCreateCanvas and nodeImage after initialization e.g. doesn't depend on the order that dependencies are included (if we tried to statically detect whether e.g. nodeCreateCanvas exists then it would depend on whatever sets global.nodeCreateCanvas to be included first for example, and it is tricky to guarantee order of initialization)

Would maybe help with issues such as #1739

[cmdcolin]

ImageBitmapType now not available statically (due to trying not to depend on initialization order). There could be alternative ways e.g. we make the ponyfill return a callback that gives us the createCanvas callback perhaps, but this is a bit intrusive.

[codecov]

Codecov Report

Merging #3044 (ffa1f7b) into main (39af675) will decrease coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main    #3044      +/-   ##
==========================================
- Coverage   61.71%   61.69%   -0.02%     
==========================================
  Files         591      591              
  Lines       26919    26922       +3     
  Branches     6527     6526       -1     
==========================================
- Hits        16612    16610       -2     
- Misses      10004    10009       +5     
  Partials      303      303              

Impacted Files	Coverage Δ
products/jbrowse-img/src/index.js	0.00% <0.00%> (ø)
products/jbrowse-img/src/index.testmod.js	0.00% <0.00%> (ø)
packages/core/util/offscreenCanvasPonyfill.tsx	51.51% <100.00%> (+1.51%)	⬆️
products/jbrowse-web/src/tests/util.js	92.72% <100.00%> (+0.27%)	⬆️
packages/core/util/analytics.ts	89.79% <0.00%> (-2.05%)	⬇️
.../linear-genome-view/src/LinearGenomeView/index.tsx	84.61% <0.00%> (-0.34%)	⬇️

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[cmdcolin]

I think it should be good to go. The only concern I see is we could more or less use a plain old polyfill for createCanvas instead of this ponyfill but that can probably be changed later if needed