Add LoadContents union type in parser

[GlassOfWhiskey]

This commit adds a LoadContents union type that contains all concrete classes that extend the LoadContents abstract class in CWL Schema SALAD.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.64%. Comparing base (f14ff6a) to head (df74bef).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #389   +/-   ##
=======================================
  Coverage   34.63%   34.64%           
=======================================
  Files          30       30           
  Lines       35398    35400    +2     
  Branches     9538     9538           
=======================================
+ Hits        12261    12263    +2     
  Misses      20257    20257           
  Partials     2880     2880           

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