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/headless/index.tsx

@@ -7,6 +7,8 @@
  * @format
  */
 
+/* eslint-disable flipper/no-relative-imports-across-packages */ // TODO T71355623
+
 import path from 'path';
 import {createStore, Dispatch, Middleware, MiddlewareAPI} from 'redux';
 import {applyMiddleware} from 'redux';