Prefer logic on the SupSub node rather than separate logic on SubscriptCommand and SuperscriptCommand

[jared-hughes]

Mathquill had separate SubscriptCommand and SuperscriptCommand node classes, which were subclasses of SupSub.

Before this PR, it was possible to create a SuperscriptCommand that only contained a subscript with the following steps:

1. Create a superscript like x^2
2. Add a subscript, x_1^2
3. Remove the superscript, x_1.

(The reverse is possible as well; you could make x^2 be a SubscriptCommand by switching superscript and subscript in the steps).

This caused an issue when you typed ) after the resulting x_1 because it duplicated the subscript to the superscript, giving x_1^1. (presumably in finalizeTree). A simple fix for that would be to do change the prototype to a SubscriptCommand, but instead this PR removes the SuperscriptCommand and SubscriptCommand classes in favor of using the supsub property on the SupSub class.

While merging the SuperscriptCommand and SubscriptCommand methods into SupSub, I found and fixed a couple more problems:

1. Mathspeak previously had an undefined show up for a SupSub with both a superscript and a subscript. E.g. for x_1^y it said "x" Subscript, 1 , Baseline "y" undefined. Now, it says "x" Subscript, 1 , Baseline Superscript, "y" , Baseline.
2. Mathspeak previously omitted the subscript for a SupSub with both a subscript and positive integer superscript. E.g. for x_1^2 it could say "x" squared, forgetting the subscript.
   • This problem only occurred when the SupSub was a SuperscriptCommand, i.e. you typed the superscript first before the subscript.