support `link` on connections

[alixander]

the text points to whatever the link is

[alixander]

In the compiler, the Link keyword is applied here. Just need to do that for Edges too. https://github.com/terrastruct/d2/blob/master/d2graph/d2graph.go#L131

And then the exporter needs to take that value from the compiler and read it, like Shape does: https://github.com/terrastruct/d2/blob/master/d2exporter/export.go#L198

And then rendered in the SVG with an tag: https://github.com/terrastruct/d2/blob/master/d2renderers/d2svg/d2svg.go#L909

[huyz]

Dup of https://github.com/terrastruct/d2/issues/1497