Custom eslint rule for disallowing cross-package references

Summary:
Added infra for writing and using custom eslint rules and created a rule for disallowing cross-package file imports. Such imports are anti-pattern and they also break standalone plugin bundling.

We still have a bunch of places where Flipper core references code directly from plugins. I've ignored these places for now and added task T71355623 to revisit them.

Reviewed By: passy

Differential Revision: D22998955

fbshipit-source-id: d04cff8fc115ba1300a7e6830306ec134046e927

desktop/plugins/databases/__test__/DatabaseDetailSidebar.node.tsx

@@ -9,6 +9,8 @@
 
 import {render, fireEvent} from '@testing-library/react';
 import React from 'react';
+// TODO T71355623
+// eslint-disable-next-line flipper/no-relative-imports-across-packages
 import reducers, {Store} from '../../../app/src/reducers';
 import configureStore from 'redux-mock-store';
 import {Provider} from 'react-redux';