Refactor the code to create a language service module

[rcjsuen]

We should create a new npm module which only includes the language services so that they can be reused in other places such as a standalone Monaco Editor. This will make it easier to embed this project in other places without having to start up a second process or necessarily support the entire language server protocol.

We created the dockerfile-utils project and moved the validator and formatter there when we implemented #79. However, I'm not sure it makes sense to move the other language services there as they're not exactly meant to be consumed by an end user on the CLI. Hence, I think a new npm module should be created instead to house the language services. We can then simply re-export the validator and formatter from dockerfile-utils there.

[rcjsuen]

I've extracted the code out into the new rcjsuen/dockerfile-language-service repository. Still need to make the necessary modifications here though.