RN: Require @flow Annotations (#51824)

Summary:
Pull Request resolved: #51824

Configures ESLint to require `flow` (or `noflow`) annotations for JavaScript files in the React Native repository. This ensures that we uphold a high bar for type safety and correctness, or intentionally deviate when it makes sense.

Changelog:
[Internal]

Reviewed By: NickGerleman

Differential Revision: D75985490

fbshipit-source-id: 56f9fe7666a02c796ceb9cf076677e81b164b65e

Author: yungsters

.eslintrc.js

@@ -4,6 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
+ * @noflow
  * @format
  */
 
@@ -27,6 +28,7 @@ module.exports = {
       files: ['*.js', '*.js.flow', '*.jsx'],
       parser: 'hermes-eslint',
       rules: {
+        'ft-flow/require-valid-file-annotation': [2, 'always'],
         'no-extra-boolean-cast': 0,
         'no-void': 0,
         // These rules are not required with hermes-eslint
@@ -68,6 +70,7 @@ module.exports = {
     {
       files: ['flow-typed/**/*.js', 'packages/react-native/flow/**/*'],
       rules: {
+        'ft-flow/require-valid-file-annotation': 0,
         'lint/valid-flow-typed-signature': 2,
         'no-shadow': 0,
         'no-unused-vars': 0,