dmdoc should handle crosslinks into the BYOND reference

[SpaceManiac]

This should be implemented by adding information to the BYOND builtin symbols about which section in the reference documents them. That table could then be used to generate links like http://www.byond.com/docs/ref/#{}.

That same table could also replace the current somewhat-fragile logic in the langserver+vscode plugin in order to embed the reference in VSC more accurately.