Create Engine API to expose

[Kiyoshika]

Currently the RenderTree is being exposed directly, but rather we should have a general Engine API that is exposed the user agents can use. This should use the DNS system recently implemented by @jaytaph

[jaytaph]

Do we have some research on how other engines expose their API? We don't need to mimic them 1 on 1, but it gives us at least an idea on what kind of communication needs to be done between the engine and a user-agent.

[Sharktheone]

I think the simplest (and maybe best) way to do this, is to just expose the rust functions and structs that are used for communicating to the engine. By that I mean, we expose the internal rust things as they are. Maybe we need a thin wrapper around them for handling things like free and so on.