parsed actions: Handle destination placeholders

[pokey]

Something like this:

big bring <user.cursorless_target> <user.cursorless_destination>:
    user.cursorless_custom_command_destination("big bring <target> <destination>", cursorless_destination, cursorless_target)

The reason to have a separate action (cursorless_custom_command_destination instead of using existing cursorless_custom_command) is that way we don't have to mix targets and destnations

We would add a new type of DestinationDescriptor of type "placeholder", and add that to out custom command grammar. I don't think it needs to be a mark, because you're not allowed to put modifiers before a destination