Fix ability to use add-track force on symlink tracks

[cmdcolin]

Possible fix for #1958

The main thing is that it needs to unlink the destination before trying to symlink it

It is somewhat of a coincidence that copy behavior is to overwrite by default. I changed it to actually use COPYFILE_EXCL to avoid accidentally overwriting a destination file, and the user can go back and use --force to overwrite it, which matches the behavior of overwriting the symlink added in this PR

It uses a try catch because if the destination symlink doesn't exist it would otherwise throw

Adds some misc refactoring (could be split out) to try to simplify or deduplicate some repetitive logic in the code and tests with the hope that simpler code==easier to see code paths

https://github.com/GMOD/jbrowse-components/compare/fix_symlink_add_track?expand=1#diff-7c20288c5391381db2117575ccf46defd61fd768480ee3af4cc60d2fe1bbde21R315-R318

[codecov]

Codecov Report

Merging #1966 (852b980) into main (9b37051) will increase coverage by 0.88%.
The diff coverage is 60.42%.

@@            Coverage Diff             @@
##             main    #1966      +/-   ##
==========================================
+ Coverage   59.27%   60.15%   +0.88%     
==========================================
  Files         459      467       +8     
  Lines       21736    21895     +159     
  Branches     5081     5103      +22     
==========================================
+ Hits        12885    13172     +287     
+ Misses       8550     8421     -129     
- Partials      301      302       +1     

Impacted Files	Coverage Δ
packages/core/ui/FileSelector.tsx	68.62% <ø> (-0.61%)	⬇️
packages/core/ui/RecentSessionCard.js	14.28% <ø> (ø)
...s/breakpoint-split-view/src/BreakpointSplitView.ts	7.69% <ø> (-1.75%)	⬇️
products/jbrowse-desktop/src/Loader.tsx	0.00% <0.00%> (ø)
products/jbrowse-desktop/src/jbrowseModel.js	15.49% <0.00%> (-2.25%)	⬇️
products/jbrowse-desktop/src/rootModel.ts	36.11% <0.00%> (-1.39%)	⬇️
products/jbrowse-web/src/AdminComponent.tsx	0.00% <0.00%> (ø)
products/jbrowse-web/src/Loader.tsx	55.63% <0.00%> (-3.44%)	⬇️
...point-split-view/src/components/Translocations.tsx	3.50% <3.50%> (-9.00%)	⬇️
products/jbrowse-desktop/src/JBrowse.js	55.17% <16.66%> (-3.01%)	⬇️
... and 42 more

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