Ability to exclude Query, Mutation and Subscription types from imported schema

This feature would be useful when we have types and inputs generated by Prisma, and want to add custom implemented Query and Mutation for real-world app.

For example:
```
const { importSchema } = require("graphql-import");
const { GraphQLServer } = require('graphql-yoga');
const { mergeTypes } = require("merge-graphql-schemas");

const typeDefs = mergeTypes([
importSchema(`# import * from './src/graphql/generated/prisma.graphql'`),
readFileSync('./src/common/schema.graphqls', {encoding: 'utf-8'}),
readFileSync('./src/auth/schema.graphqls', {encoding: 'utf-8'}),
...
], { all: true });

const server = new GraphQLServer({
typeDefs,
...
});
```

But now I can't do that because `importSchema` grabs generated Query and Mutation types directly into application API.

Start at the importSchema entry point and inspect how it imports the generated Prisma schema, including its Query, Mutation, and Subscription types. Compare the imported schema with the custom schemas passed to mergeTypes; done means callers can exclude selected root operation types while retaining the generated types and inputs they need.

Written by the indexing model from the issue text.