delete \vector

it was an ancient experimental attempt at LaTeX's
\begin{array}...\end{array}, is terrible, and is getting in the way of
progress towards refactors that will actually enable doing environments
like array cleanly, like #186 `refactor.key-events`
Merge branch 'feature.rm-vector' into refactor.key-events

because fixing Vector::onKey is too much trouble.

Conflicts:
	src/commands.js