Update handling of braket bars so that the enclosing braket can be more accurately found. (mathjax/MathJax#3164)

[dpvc]

This PR fixes a problem where the BraketMethods.Bar() function doesn't find the enclosing braket item properly when there are certain types of enclosing elements (like style or color changes).

To do this, we add a function to the Stack object to get its current height (the number of elements on the stack), and save that in the braket item's env object, which will be available in the parser stack's env object. Then the BraketMethods.Bar() function can use parser.stack.env.braketItem to find the matching braket item without having to look through the stack items to find it. (The current approach doesn't work if there are intervening items other than an over item).

We also set the braket package priority so that it is searched before the base package, guaranteeing that the braket function for | is used rather than the base one.

Finally, we fix a few comments along the way.

Resolves issue mathjax/MathJax#3164.

[zorkow]

lgtm.