[graphql-language-service-interface] add suggestions to spread fragments on Field

Right now fragment spread suggestions are shown only when you start with `...` fragment spread operator.

I'd like to propose adding fragment spread suggestion earlier (on `Field`, `SelectionSet` etc), so we it's much easier for new GraphQL users to discover them.

Basic idea is to have it look something like:

Another suggestion from me would be to consider adding inline fragments to union types as well:
(unions and inline fragments are quite common pain points for [Gatsby](https://github.com/gatsbyjs/gatsby) users that are new to GraphQL)

This would require just slight refactoring to reuse already existing code and I'm happy to submit pull request if this change would be welcome here.