Added the checker as a property of the language service

This is to mainly get a feel for the new API, before augmenting the
language service
Added initial cut of highlight usages/ find references

At the moment only highlight usages is working and there are a number
of issues to resolve.

Currently there is quite a lag at startup as the full project is parsed.

Some uses of long indents are not working correctly e.g.
Thing1.Thing2.Thing