[TeX] support for \hdotsfor (from amsmath)

[pkra]

We had a first use case for this recently. It would be great to have this in MathJax's amsmath implementation.

[dpvc]

I think that would require the rowspan/columnspan support, which we don't yet have. I'm also not sure what MathML structure might correspond to a variable number of dots, as there is nothing like TeX's "leaders" that I am aware of.

A by-hand work-around might be to use \rlap{\text{. . . . . . . . . .}} in the first column and pick the right number of dots by hand. A hack, but could be used in a pinch.