Bug 1790009, part 2 - Operator dictionary: Only accept UTF-16 strings of 1 or...

Bug 1790009, part 2 - Operator dictionary: Only accept UTF-16 strings of 1 or 2 characters. r=emilio

MathML Core specifies that operators containing a UTF-16 strings whose
length is not 1 or 2 should use the default properties [1]. This
commit removes the obsolete strings of length 3 from our operator
dictionary and tweak updateOperatorDictionary.pl to ensure it only
accepts strings of 1 or 2 characters. This also adds an early return
in LookupOperator to immediately fallback to default properties.

[1] https://w3c.github.io/mathml-core/#dfn-algorithm-to-determine-the-category-of-an-operator

Differential Revision: https://phabricator.services.mozilla.com/D157706