`Variable_handler_command` and `Variable_handler_action` provide a way of representing commands whose keys, description and group are constant, but whose handler varies with the `'env` variable.

This is useful because it provides a way of generating help text for these commands that does not depend on or vary with the `'env` variable.

`type 'env t````=` `{`

`keys : Keystroke.t list;`
`description : string;`
`group : Grouped_help_text.Group_name.t option;`
`handler : 'env ‑> Keyboard_event_handler.Handler.t;`

`}```
`val get_help_text : _ t ‑> Help_text.Command.t`