Allow sorting only top level declarations in schema

**Is your feature request related to a problem? Please describe.**

This library is a dependency of graphql-code-generator which is used to produce a generated schema file.

https://github.com/dotansimha/graphql-code-generator/issues/6935

I opened an issue on graphql-js here https://github.com/graphql/graphql-js/issues/3971 but it was closed due to there being a trivial work-around. We can use this approach in this library to resolve my issue.

**Describe the solution you'd like**

Add a field to the options on `loadSchemaSync` which enables sorting only the top level declarations

```js
return options.sort
? lexicographicSortSchema(sources[0].schema)
: options.sortShallow // naming?
? shallowSortSchema(sources[0].schema)
: sources[0].schema;

...

function shallowSortSchema(schema) {
const schemaConfig = schema.toConfig();
const sortedSchema = new GraphQLSchema({
...schemaConfig,
types: [...schemaConfig.types].sort((a, b) => a.name.localeCompare(b.name)),
});
}
```

I can prepare a PR for this myself.

**Describe alternatives you've considered**

Currently I am applying a patch to the graphql-js lexicographicSortSchema function in order to produce my desired output.

**Additional context**

Start at the loadSchemaSync options and the existing lexicographicSortSchema path described in the issue. Check how GraphQLSchema is rebuilt from its config, then add an option that sorts only top-level declarations while preserving nested ordering; done means graphql-code-generator can use the new behavior without patching graphql-js.

Written by the indexing model from the issue text.