Add ESLint rule playground (facebook#14609)

* Add ESLint rule playground

* Update index.js

* Update index.js

fixtures/eslint/.eslintrc.json

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+{
+  "root": true,
+  "parserOptions": {
+    "ecmaVersion": 6,
+    "sourceType": "module",
+    "ecmaFeatures": {
+      "jsx": true
+    }
+  },
+  "plugins": ["react-hooks"],
+  "rules": {
+    "react-hooks/rules-of-hooks": 2
+  }
+}

fixtures/eslint/README.md

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+# ESLint Playground Fixture
+
+This is an internal playground for quick iteration on our lint rules inside an IDE like VSCode.
+
+See instructions in `./index.js` in this directory.
+
+![Demo](https://duaw26jehqd4r.cloudfront.net/items/2Z390a31003O0l0o0e3O/Screen%20Recording%202019-01-16%20at%2010.29%20PM.gif?v=d6856125)

fixtures/eslint/index.js

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+// This is a testing playground for our lint rules.
+
+// 1. Run yarn && yarn start
+// 2. "File > Add Folder to Workspace" this specific folder in VSCode with ESLint extension
+// 3. Changes to the rule source should get picked up without restarting ESLint server
+
+function Foo() {
+  if (condition) {
+    useEffect(() => {});
+  }
+}

fixtures/eslint/package.json

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+{
+  "private": true,
+  "name": "eslint-playground",
+  "dependencies": {
+    "eslint": "4.1.0",
+    "eslint-plugin-react-hooks": "link:./proxy"
+  },
+  "scripts": {
+    "start": "./watch.sh",
+    "lint": "eslint index.js"
+  }
+}

fixtures/eslint/proxy/index.js

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+'use strict';
+
+// This file is a proxy for our rule definition that will
+// load the latest built version on every check. This makes
+// it convenient to test inside IDEs (which would otherwise
+// load a version of our rule once and never restart the server).
+// See instructions in ../index.js playground.
+
+let build;
+reload();
+
+function reload() {
+  for (let id in require.cache) {
+    if (/eslint-plugin-react-hooks/.test(id)) {
+      delete require.cache[id];
+    }
+  }
+  // Point to the built version.
+  build = require('../../../build/node_modules/eslint-plugin-react-hooks');
+}
+
+let rules = {};
+for (let key in build.rules) {
+  if (build.rules.hasOwnProperty(key)) {
+    rules[key] = Object.assign({}, build.rules, {
+      create() {
+        // Reload changes to the built rule
+        reload();
+        return build.rules[key].create.apply(this, arguments);
+      },
+    });
+  }
+}
+
+module.exports = {rules};

fixtures/eslint/proxy/package.json

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+{
+  "private": true,
+  "version": "0.0.0"
+}

fixtures/eslint/watch.sh

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+#!/bin/bash
+(cd ../.. && yarn build eslint --type=NODE_DEV)
+(cd ../.. && watchman-make --make 'yarn build eslint --type=NODE_DEV' -p 'packages/eslint-plugin-*/**/*' -t ignored)