[dev-context] \lastxpos and \lastypos should behave like registers

Dear devs, The two macros \lastxpos and \lastypos should behave like registers, because that is what they are in LuaTeX. The following MWE fails with ! You can't use `the character 0' after \the \starttext \the\lastxpos \the\lastypos \stoptext A simple fix would be to wrap the macro in \numexpr like in the attached patch. Cheers, Henri