Expose symbols API through $psEditor model

[daviwil]

This API will allow $psEditor users to request symbols for the current document with which they can perform operations. This should also allow registering new "symbol providers" which are capable of scanning for symbols that PSES is not aware of (like custom DSLs, etc).

/cc @nightroman

[daviwil]

I thought I had implemented this one fully but it turns out there's no way to get a ScriptFile instance directly from the $psEditor API. I'll hold off on the completion of this task until 1.4 because we should have the new IWorkspace and ITextDocument interfaces implemented.