Fix issue with using --force error when no track was previously loaded

[cmdcolin]

If --force was used but no track/file existed there to overwrite, then the script failed

This checks for the file existing

This code is a little tedious and it would be sorta nice if we could "operate at a slightly higher level of abstraction" than the raw fs module, but this does fix the issue :)

[codecov]

Codecov Report

Merging #2061 (41def8b) into main (34a2ec7) will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##             main    #2061   +/-   ##
=======================================
  Coverage   61.27%   61.27%           
=======================================
  Files         480      480           
  Lines       22981    22981           
  Branches     5271     5271           
=======================================
  Hits        14082    14082           
  Misses       8624     8624           
  Partials      275      275           

Impacted Files	Coverage Δ
products/jbrowse-cli/src/commands/add-track.ts	53.27% <66.66%> (ø)
packages/core/util/index.ts	78.30% <0.00%> (ø)

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