Add :vs ex mode command as a synonym for :vsplit

[sethfowler]

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Here are the docs for vim's :vsplit command:

:[N]vs[plit] [++opt] [+cmd] [file]          *:vs* *:vsplit*
        Like |:split|, but split vertically.  The windows will be
        spread out horizontally if

As you can see, :vs is listed in the documentation as the short form of :vsplit, but vscodevim currently doesn't support :vs - instead, you have to type :vsp or :vsplit. It'd be really nice to have :vs too, as it's hardwired into my muscle memory at this point.