Create concept of global hovered state

[cmdcolin]

This creates an object session.hovered, similar to session.selected

The hovered state is rapidly updated by all onMouseMove events from LinearGenomeView, and includes hoveredPosition (result of pxToBp of the current mouseover x coordinate) and hoveredFeature which is the result of searching through view.tracks.find(t=>t.displays[0].featureUnderMouse)

• This hover state is updated in a single place, which reduces the need to wire the code through many track types.

• The "downside" is that it is always performing that views.tracks.find call instead of some more targeted session.setHovered right at the point-of-setting the featureUnderMouse but that seems like a hypothetical concern in terms of performance

• Any track type that wishes to opt in should make the top level display have the featureUnderMouse getter (displays with subviews like alignments display can "lift this state up")

This hover state can be observed by potential applications like the msaview and protein view and to translate the genome hoveredPosition to protein coordinates

Possible enhancements could include 'sub-feature mouseovers' so that the plugin would be aware of the exact exon being mouse-overed, etc.

Interested in feedback

[cmdcolin]

code for the single hover change https://github.com/GMOD/jbrowse-components/pull/4013/files#diff-4048d8cd75237f43af0b24916d1bcb3a7a9eb5be7728706b83a57083176eed5dR101-R107

[codecov]

Codecov Report

Merging #4013 (29e54fa) into main (479e97e) will increase coverage by 0.00%.
The diff coverage is 91.66%.

❗ Current head 29e54fa differs from pull request most recent head 824c5b0. Consider uploading reports for the commit 824c5b0 to get more accurate results

@@           Coverage Diff           @@
##             main    #4013   +/-   ##
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  Coverage   63.67%   63.67%           
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  Files        1026     1026           
  Lines       30232    30245   +13     
  Branches     7197     7198    +1     
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+ Hits        19249    19260   +11     
- Misses      10818    10820    +2     
  Partials      165      165           

Files	Coverage Δ
packages/core/util/types/index.ts	62.38% <ø> (ø)
packages/product-core/src/Session/BaseSession.ts	80.64% <100.00%> (+1.33%)	⬆️
plugins/svg/src/SvgFeatureRenderer/configSchema.ts	50.00% <ø> (ø)
...c/LinearGenomeView/components/LinearGenomeView.tsx	91.11% <90.00%> (-0.32%)	⬇️

... and 3 files with indirect coverage changes

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[cmdcolin]

xref #2458

[cmdcolin]

some early results show that this PR can be leveraged to get the desired "hover over genome, get position in msaview" type behavior

[cmdcolin]

i think this should be good to go, but one thing I'll randomly note after attending icn3d tutorial from ncbi is they have very sophisticated notions of "selections". sort of tangential to this pr concerning hover, but i found it interesting as it does enable various workflows in their app