feat: use new GraphQL Config

[acao]

re-created @ardatan's #1225 PR, as things got kinda tricky post typescript migration and removing the watchman

---

• Use new GraphQL Config
• Remove extra endpoint extension

About endpoint

In new GraphQL Config, there is no extension called endpoint so all sources of schema are defined as schema in GraphQL Config file.

Related #1204 (comment) and #1191

This alpha version of GraphQL Config supports GraphQL-js v15. We're testing our graphql-toolkit which has core logic in the CLI for both v14 and v15 in the CI.
https://github.com/ardatan/graphql-toolkit/pull/403/checks?check_run_id=387927623

Related #920

Also user can add their own custom schema loaders in two different ways;

schema:
    - your-own-schema-pointer:
         loader: my-loader#loaderFn

or the developer can add their loaders in the library code;

const MyOwnExtension: GraphQLExtensionDeclaration = api => {
  // Schema
  api.loaders.schema.register(new CodeFileLoader());
  api.loaders.schema.register(new GitLoader());
  api.loaders.schema.register(new GithubLoader());
  api.loaders.schema.register(new ApolloEngineLoader());
  api.loaders.schema.register(new PrismaLoader());

  // Documents
  api.loaders.schema.register(new CodeFileLoader());

  return {
    name: 'my-own-extension',
  };
};
const config = await loadConfig({ extensions: [MyOwnExtension]});

More info; https://graphql-config.com/docs/loaders'

Related #1118

Let's say you have some custom extensions in your configuration file;

schema: some-schema-pointer.graphql
extensions:
   my-own-extension:
       myOwnField: true

You can get like below

const { myOwnField } = config.get('my-own-extension');

[acao]

so excited about these new features! feels like so much is possible. thanks for all this @ardatan and @Urigo. let me know when you all go stable but there's no rush, as you'll likely have 3.0.0 stable well before we are making stable releases again

[Urigo]

That's awesome! thank you @acao !
We'll get you a stable version this week!