Make using --out for add-assembly create output directory if it does not exist and fix outputting to symlink

[cmdcolin]

Fixes #1898

Also fixes issue trying to use --out on a symbolic link, use fstat instead of lstat

Currently on main using a symlink, if i am testing correctly, produces

mkdir ~/testing
ln -s testing
bin/run add-assembly --out testing volvox.fa --load copy
    Error: ELOOP: too many symbolic links encountered, open 'testing'
    Code: ELOOP

[codecov]

Codecov Report

Merging #1901 (0bffcdf) into main (2660153) will decrease coverage by 0.00%.
The diff coverage is 64.10%.

@@            Coverage Diff             @@
##             main    #1901      +/-   ##
==========================================
- Coverage   58.52%   58.51%   -0.01%     
==========================================
  Files         460      460              
  Lines       21290    21311      +21     
  Branches     5059     5066       +7     
==========================================
+ Hits        12459    12470      +11     
- Misses       8526     8536      +10     
  Partials      305      305              

Impacted Files	Coverage Δ
packages/core/ReExports/modules.ts	75.00% <ø> (ø)
packages/core/ui/DrawerWidget.js	93.10% <ø> (ø)
...a-management/src/AssemblyManager/AssemblyTable.tsx	82.35% <ø> (ø)
plugins/wiggle/src/index.ts	100.00% <ø> (ø)
.../core/BaseFeatureWidget/SequenceFeatureDetails.tsx	53.45% <44.00%> (-3.30%)	⬇️
products/jbrowse-cli/src/commands/add-assembly.ts	90.86% <100.00%> (+0.16%)	⬆️
...ts/jbrowse-cli/src/commands/set-default-session.ts	96.36% <100.00%> (+0.44%)	⬆️

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[cmdcolin]

I think this should be good to go, pretty small:)