scheme: Add is_definition relationship

[varungandhi-src]

Without this, modeling multiple inheritance (or single inheritance with mixins) is unnecessarily painful.

Test plan

• Going to see if the code navigation works as expected in an SG instance with the new relationships statement in repro, which allows adding a relation to a symbol which lacks a definition.

[varungandhi-src]

Looks like this does have the desired effect. 🎉

[olafurpg]

LGTM.

I see the hover is empty, should we add is_hover as well?

[varungandhi-src]

IIUC, the hover is empty because I set the documentation to be blank. So I don't quite get what is_hover would solve... If A is a definition for B, isn't the desired behavior that you show hover docs of A when you hover over B? Or you could supply hover docs for B in B's symbol information if you wanted that to be the case.

[varungandhi-src]

Let's merge this for now so that I can start using it in scip-ruby. We can add is_hover later too.