parser/printer: Add support for parsing :command
parser/printer: Do not parse last :command argument

Reasoning: things like <args> need to be expanded before parsing anything. I can
redirect parsing of some things like <f-args> to expressions parser or <line1>
to range parser, but this is not compatible and is not going to work for things
like

    command -nargs=1 Sil silent! <args>
tests: Add tests for :command parser