[DevTools] Add fb local build command (facebook#32644)

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## Summary
1. Having a development build for FB will be convenient for fb internal
feature development
2. Add a new checkbox to toggle new internal features added to React
Devtools.

## How did you test this change?
1. yarn test
2. set extra env variables in bash profile and build an internal version
with the new script.
3. toggle on/off the new checkbox, the value is stored in local storage
correctly.

---------

Co-authored-by: Aohua Mu <muaohua@fb.com>

Author: aohua

packages/react-devtools-extensions/package.json

@@ -8,6 +8,7 @@
     "build:chrome": "cross-env NODE_ENV=production node ./chrome/build",
     "build:chrome:fb": "cross-env NODE_ENV=production FEATURE_FLAG_TARGET=extension-fb node ./chrome/build --crx",
     "build:chrome:local": "cross-env NODE_ENV=development node ./chrome/build",
+    "build:chrome:fb:local": "cross-env NODE_ENV=development FEATURE_FLAG_TARGET=extension-fb node ./chrome/build",
     "build:firefox": "cross-env NODE_ENV=production node ./firefox/build",
     "build:firefox:local": "cross-env NODE_ENV=development node ./firefox/build",
     "build:edge": "cross-env NODE_ENV=production node ./edge/build",

packages/react-devtools-shared/src/devtools/views/Settings/GeneralSettings.js

@@ -12,6 +12,7 @@ import {useContext, useMemo} from 'react';
 import {SettingsContext} from './SettingsContext';
 import {StoreContext} from '../context';
 import {CHANGE_LOG_URL} from 'react-devtools-shared/src/devtools/constants';
+import {isInternalFacebookBuild} from 'react-devtools-feature-flags';
 
 import styles from './SettingsShared.css';
 
@@ -46,6 +47,12 @@ export default function GeneralSettings(_: {}): React.Node {
 
   return (
     <div className={styles.Settings}>
+      {isInternalFacebookBuild && (
+        <div className={styles.Setting}>
+          This is an internal build of React DevTools for Meta
+        </div>
+      )}
+
       <div className={styles.Setting}>
         <div className={styles.RadioLabel}>Theme</div>
         <select