Add export option for single-scale OME-Zarr

[btbest]

Instead of adding a triplicate of the H5/N5 file options widget, I figured I'd combine them instead. Still pondering whether the one point of custom logic for zarr (allowing dataset to be empty) warrants a duplicate, but don't really think so. Not sure why it's mandatory anyway; there is ilastik code that enforces an internal path to be present for H5 but not N5.

I've put the menu option under the below hierarchical formats for now, but that's easy to relocate if we want to have it further up or down the list.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 25 lines in your changes missing coverage. Please review.

Project coverage is 36.48%. Comparing base (baf0c13) to head (5292923).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
...ina/widgets/hierarchicalFileExportOptionsWidget.py	0.00%	18 Missing ⚠️
.../widgets/multiformatSlotExportFileOptionsWidget.py	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #316      +/-   ##
==========================================
- Coverage   36.58%   36.48%   -0.10%     
==========================================
  Files         108      107       -1     
  Lines       11489    11371     -118     
==========================================
- Hits         4203     4149      -54     
+ Misses       7286     7222      -64     

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[k-dominik]

Cool, nice refactor, too :) great to unify those things :). And less code, more functionality - you seem to have a knack for that 👍

Could've been rebased onto main before opening... (I thought darkmode was broken again...)

[btbest]

Ready to merge when ilastik PR is done