Make non-erroring ether validation operators

[lgessler]

Problem

Sometimes it's useful to have an ether validation operator just highlight errant cells but not treat it as a reportable validation error. This is not the behavior of any of the current ether validators.

Proposed solution

Make a variant of each operator that is highlight-only, with some sort of consistent notation. E.g., for the equal span and content operator ==, the variant might be ==? or (==).

To do this, ether_validator.py would need to be modified, and the new tokens would need to be added to the validation rules interface

[amir-zeldes]

One other aspect of this is the existence of the column: currently even in the non-error deployment on ling367, if there is a rule:

tok == tok_orig

and tok_orig does not exist, highlighting appears on every single token. For == the intention is probably not to flag cases where the column does not exist (that's what exists is for), but maybe this is also something we need to parametrize.