Add Intellisense for the Interaction Language

[alistairjevans]

We've got intellisense for test files, now we need it for interactions.

This may be a bit more complicated, but we'll see how we go.

[alistairjevans]

Position Indexing has been added to AutoStep for the interactions language; now we need to do the handling in the language server.

[alistairjevans]

Partial implementation complete; but still need go to definition support, and completion for available traits.