Add Link definition node to the AST

[chrisjsewell]

For HTML rendering, this is not necessary, since it is not rendered. But for use cases, such a "round-trip" rendering (MD -> MD), or syntax analysis tools, it would be necessary to have this present in the AST.

(similarly capturing the original labels of link references would also be required but, if accepted, I can add that in a separate PR)

[rlidwka]

Looks good to me.

Not sure about name, what are the chances that Definition is gonna collide with something like footnote definitions?