`monaco-graphql` should export JSONSchema

To empower @jonathanawesome 's proposed `EasyVars`, inspired by onegraph's variables form, as well as other tools that want to use it, `monaco-graphql` should expose the current variables JSONSchema via an API and change events.

This way it could be:

- used to generate complete forms from JSON Schema using any of the number of react form library support
- used with `monaco-yaml` and perhaps other monaco modes that support the `monaco-json` like `JSONSchema` `schemas` option.
- used for other forms of validation or interaction, ala vscode api "CustomEditors" - for example, a hypothetical platform could implement a drag and drop-enabled 'variables' pane with a custom renderer that, as changes occur, updates the string value from AST being generated/modified in memory