Link to deno runtime types from external files

[lucacasonato]

So make the Deno.ListenOptions type clickable and redirect to https://doc.deno.land/https/github.com/denoland/deno/releases/latest/download/lib.deno.d.ts#Deno.ListenOptions. This can be handled fully client side.

A related super complicated thing would be to make the Reader type clickable and redirect to https://doc.deno.land/https/github.com/denoland/deno/releases/latest/download/lib.deno.d.ts#Deno.Reader. This is super complicated because Reader is actually an alias to Deno.Reader. The only really possible way of doing this (which would be complex) is for deno doc to add a field to all type_ref TS types which would give the resolved global scope location to that type (so that Reader would have Deno.Reader as its resolved type)

/cc @bartlomieju