Make over/under braces and matrices be full size, as in actual TeX (mathjax/MathJax#3300)

[dpvc]

This PR fixes some issues with the size of the contents of \overbrace, \underbrace, and matrices so that they match the output of actual LaTeX. The argument to the over and under brace macros is actually typeset in display style in LaTeX, and the matrices are in text mode.

The changes to MmlNode.ts fix a problem with the inherited values of displaystyle and scriptlevel so that they don't produce _inherit_ for example, which is supposed to be translated into the actual inherited value. This allows the new filter discussed below to work.

The smallmatrix and subarray environments used to use a scriptlevel property to tell mtable to use a different script level, but that is now handled by wrapping the tables with mstyle elements that set scriptlevel explicitly, and the property is changed to smallmatrix so, which affects the interline spacing, and needs to be preserved through a data-mjx-smallmatrix attribute. A similar mstyle is added for the brace macros.

I hate to see extra mstyle tags that are unnecessary, however, so I've also added another post-filter to the TeX input jax that removes unneeded ones (when the scriptlevel and displaystyle match the surrounding values). This also removes unneeded mstyle nodes from the spacing macros like \,, but keeps them when they are needed.

If you don't like the filter, that can be removed, and the mstyle nodes can be kept.

Resolves issue mathjax/MathJax#3300.

[zorkow]

lgtm.
But see my comment on BaseMethods.ts for consideration. Going forward, should we go for more concise code or for testable paths?

[zorkow]

Could be done with a ternary expression of the form:

Suggested change

	array.arraydef['scriptlevel'] = 0;
	array.arraydef['scriptlevel'] = style === 'S' ? 1 : 0;

However, this way we get two test paths for completeness.

[dpvc]

should we go for more concise code or for testable paths?

I'm for more concise code, to keep the bundle size down. While the coverage information is useful, it really doesn't mean that all the needed cases are actually covered, so I still think we need to look carefully at the code to produce a complete set of tests, so I would say we would pick this up then.