Add typescripting to hierarchical track selector

[cmdcolin]

Begins adding a little type safety to hierarchical track selector, which has long been un-typescripted :)

[codecov]

Codecov Report

Merging #3035 (6fa8420) into main (cb6f2f0) will increase coverage by 0.06%.
The diff coverage is 64.23%.

@@            Coverage Diff             @@
##             main    #3035      +/-   ##
==========================================
+ Coverage   61.64%   61.70%   +0.06%     
==========================================
  Files         590      590              
  Lines       26924    26920       -4     
  Branches     6522     6528       +6     
==========================================
+ Hits        16598    16612      +14     
+ Misses      10020    10005      -15     
+ Partials      306      303       -3     

Impacted Files	Coverage Δ
...electorWidget/components/CloseConnectionDialog.tsx	0.00% <0.00%> (ø)
...lectorWidget/components/DeleteConnectionDialog.tsx	0.00% <ø> (ø)
...ectorWidget/components/ManageConnectionsDialog.tsx	0.00% <ø> (ø)
...ectorWidget/components/ToggleConnectionsDialog.tsx	0.00% <0.00%> (ø)
...ement/src/HierarchicalTrackSelectorWidget/index.ts	100.00% <ø> (ø)
...roducts/jbrowse-desktop/src/sessionModelFactory.ts	6.25% <0.00%> (+0.11%)	⬆️
...ement/src/HierarchicalTrackSelectorWidget/model.ts	67.79% <64.91%> (ø)
...torWidget/components/HierarchicalTrackSelector.tsx	64.17% <76.31%> (ø)
packages/core/util/types/index.ts	68.60% <100.00%> (+0.74%)	⬆️
... and 2 more

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