Read new JWST WFSS file formats

[rosteen]

JWST is changing some of its data products to store the spectra from multiple sources in a single FITS extension rather than having one extension per source for WFSS data.

[codecov]

Codecov Report

Attention: Patch coverage is 93.61702% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.80%. Comparing base (9de8167) to head (2710be0).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
specutils/io/default_loaders/jwst_reader.py	93.61%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1252      +/-   ##
==========================================
+ Coverage   86.59%   86.80%   +0.21%     
==========================================
  Files          63       63              
  Lines        4722     4729       +7     
==========================================
+ Hits         4089     4105      +16     
+ Misses        633      624       -9     

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