mrow-like elements and determination of postfix/prefix form

[fred-wang]

From https://mathml-refresh.github.io/mathml-core/#dictionary-based-attributes

• If the embellished operator is the first in-flow child of an unknown MathML element, math, menclose, merror, mpadded, mphantom, mrow, msqrt or mstyle with more than one in-flow child (ignoring all space-like children) then it has form prefix;
• Or, if the embellished operator is the last in-flow child of an unknown MathML element, math, menclose, merror, mpadded, mphantom, mrow, msqrt or mstyle with more than one in-flow child (ignoring all space-like children) then it has form postfix;

With the proposal in #183, this can be rewritten:

• If the embellished operator is the first in-flow child of an mrow-like element, menclose, mpadded, msqrt with more than one in-flow child (ignoring all space-like children) then it has form prefix;
• Or, if the embellished operator is the last in-flow child an mrow-like element, menclose, mpadded, msqrt with more than one in-flow child (ignoring all space-like children) then it has form postfix;

Which would add maction, semantics, mprescripts, and none into the list. The two first generally have only one in-flow child while the two last are empty so I think it is safe to make this change. This will simplify spec, implementation and maybe tests.

[fred-wang]

consensus from feb 3: approved

[fred-wang]

closing since the spec was updated