Implement an interlinking short-hand syntax

[machow]

Based on discussion in #315, it would be nice to allow a interlinks short-hand, to shorten links like [](`target`).

The main reason for this is that the longform is useful for customizing display name (e.g. [some name](`target`)), but cumbersome otherwise.

It seems like two important factors are:

• Easy and quick to use.
• Does not conflict too much with things people want rendered differently in quarto.
  • E.g. if we use `:target` as a shorthand, then it will become hard to represent the literal code block :target.
  • I'm onboard with a little overlap / conflict, if it makes life a lot nicer ;)

[machow]

@schloerke brought up :`target` seeming nice--but if we're worried about conflict we could use the very distinct ::`target` with two colons.