Allow non-equation inputs for nuclear questions

[sjd210]

The checker works recursively so inherently supports statements or terms while checking. This feedback was the only remaining place where we indicate that this isn't supported (even including tests and our tooltip example inputs).

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 39.87%. Comparing base (abe0f55) to head (88490c3).

Files with missing lines	Patch %	Lines
...tg/isaac/quiz/IsaacSymbolicChemistryValidator.java	0.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #767   +/-   ##
=======================================
  Coverage   39.87%   39.87%           
=======================================
  Files         547      547           
  Lines       23909    23906    -3     
  Branches     2873     2872    -1     
=======================================
  Hits         9533     9533           
+ Misses      13470    13467    -3     
  Partials      906      906           

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