Function: has_link

--Usage--
> has_link(input: card, linked_relation: "link type")

Inspects a given [[type:card]]'s links to find one of the given type.
Returns true if such a link was found, false otherwise.

Note that this function does not check if the linked card exists in the set. For that, use get_cards_from_link.

--Parameters--
! Parameter	Type				Description
| @input@	[[type:card]]		The card whose links we'll inspect.
| @linked_relation@	[[type:string]]		The type of link we want to find.