Feature: Schema editing support for localSchemaFile

Originally from: https://github.com/apollographql/vscode-graphql/issues/247

When working with localSchemaFile it would be nice to have minimal schema editing features.

Here's a few ideas that comes to mind
- "Go to Definition" when hovering on a field's type
- Basic validation like showing an error when the type of a field is unknown
- Autocomplete when typing a field's type

As mentioned in the original issue, maybe having a rover config would help, but I'd still love some support considering the extension has all the necessary schema information to provide an improved developer experience
https://www.apollographql.com/docs/graphos/schema-design/connectors/vs-code

For reference in https://marketplace.visualstudio.com/items?itemName=GraphQL.vscode-graphql
I get type validation, type information on hover and go to definition

No files, tests, or entry points are named. Start by tracing how localSchemaFile is handled in the extension and compare the requested behavior with the linked GraphOS and marketplace references. Done requires an agreed scope and working schema navigation, validation, and completion behavior for local schema files.

Written by the indexing model from the issue text.