config rules do not match TypeScript types for requiring schema property

[asakusuma]

Describe the bug

The JSON config rules for the schema technically do not require the schema field: https://github.com/kamilkisiela/graphql-config/blob/master/src/project-config.ts#L21

This is technically a legal config:

{
  "schemaPath":"foo",
  "projects":{
    "myproject": {
      "documents":["foo"]
    }
  }
}

But the types assume that every project has a defined schema field:
https://github.com/kamilkisiela/graphql-config/blob/master/src/types.ts#L53

I also don't think the JS API also has no way of accessing the schemaPath field. As their is no default project here AND project.getSchemaSync() doesn't work here.

To Reproduce
Steps to reproduce the behavior:

https://www.jsonschemavalidator.net/s/fbNGYrz8

I would not expect this config to be valid based on the typescript types

Environment:

• OS:
• GraphQL Config Version:
• NodeJS:

Additional context