feature request: shorthand issue/pull cross-ref syntax #361

it'd be nice to be able to have something similar to the way #123 autolinkifies in other git forges. given that pulls and issues don't seem to share a unified numbering scheme like in some other forges, this'd probably have to include a kind identifier.

you could require markdown reference links, similarly to how rustdoc uses them, which would make searching for/parsing a bit easier/less ambiguous. something like:

• [issues#123] --> current repo, issue #123
• [pulls#123] --> current repo, pull #123
• [@tangled.org/core/issues#123] --> specific repo, issue #123