review overbracket, underbracket (and maybe others)

[pkra]

Much like #23, our definition isn't quite right (lacking the "stack" boolean for mathjax's UnderOver method).

However, it turns out mathjax now has a core macro providing these - from the mathtools extension. In addition, mathtool's definition is quite a bit more involved:

https://github.com/mathjax/MathJax-src/blob/f939f2ff2c4678785e9bf3d3b17e8c6151f0ce9c/ts/input/tex/mathtools/MathtoolsMethods.ts#L289-L312

We could consider if and how to revise it here.

[pkra]

(lacking the "stack" boolean for mathjax's UnderOver method).

To clarify, I wrote this because IIUC

https://github.com/mathjax/MathJax-src/blob/f939f2ff2c4678785e9bf3d3b17e8c6151f0ce9c/ts/input/tex/mathtools/MathtoolsMethods.ts#L310

does what "1"

https://github.com/mathjax/MathJax-src/blob/8565f9da973238e4c9571a86a4bcb281b1d98d9b/ts/input/tex/base/BaseMappings.ts#L514

Then again, I'm not sure if unicode-math has these working this way cc @davidmjones

[pkra]

Oh, and certainly the constructions here don't work well with the default mathjax v3 fonts. Not sure about v4.