Add support for \dotminus

[roife]

KaTeX already supports \dotplus, and I want to know if we can support its counterpart \dotminus (∸, U+2238).

Although a similar symbol can be constructed using \dot - ($\dot -$), the dot constructed this way is "too high". Or could we consider using a macro to add the symbol? If possible, I would be glad to submit a PR to add this symbol.