Make schema argument optional in getAutocompleteSuggestions function

This would enable code editors to give suggestions of top-level keywords like `query`, `mutation`, etc. when the cursor is at `kind === 'Document'`.

None of the logic till then (correct me if I am missing something) uses schema.

https://github.com/graphql/graphql-language-service/blob/8345059945fade19e4cb78849034faaa947ff1af/packages/interface/src/getAutocompleteSuggestions.js#L53-L83

**I can create a PR for this if you want** 😄. Let me know.