Add `user dm` command

[SheepTester]

The alias help text gives /alias set hi "user dm -m \"Hello!\" -2 " as an example, but user dm doesn't exist.

Maybe there can also be a user executor that stores the @mention of the user who sent the message in an envvar:

@wrapper
  @main
    user executor -> me
    data log "I agree with the great and logical $(me)!"
  data run "batch $(main)" --
alias set agreewme "batch $(wrapper)"

Speaking of data run, which is used to pass arguments as envvars to an aliased batch command, there should be a version where a sequence of names is given, then after a -- the values will be assigned in order. For example,

@wrapper
  @main
    data -a dividend / -b divisor -> quotient
    data log "$(dividend) / $(divisor) = $(quotient)
  data params dividend divisor -c "batch $(main)" --
alias set divide "batch $(wrapper)"

where calling divide 6 3 would output 6 / 3 = 2

This can be used to pass multiple variables into an aliased batch command (the workaround for defining custom commands lol)