feat: add snippets

[lucacervello]

Closes #59

[mhanberg]

Another thing is you need to preserve the documentation in the completion item in the snippet

[lucacervello]

Should I add a new parameter to the Snippet.get/2 function or use the second one as docs?
Or we don't want Snippet.get/2 to change?

[mhanberg]

Doing a Map.merge(original,snippet) might do the trick, no need to change the snippet function

[mhanberg]

I'll try and play with this soon, currently wrapping up some work on the editor extensions.

[mhanberg]

made a couple changes, otherwise great!