Add RCTDevSettings module

Summary:
This decouples non-UI logic from RCTDevMenu into a new module RCTDevSettings.

**Motivation**: This allows developers to change dev settings without depending on the built-in dev menu, e.g. if they want to introduce their own UI, or have other devtools logic that doesn't depend on an action sheet.

It also introduces the RCTDevSettingsDataSource protocol for storing dev tools preferences. This could allow a developer to implement alternative behaviors, e.g. loading the settings from some other config, changing settings based on the user, deciding not to persist some settings, or something else.

The included data source implementation, RCTDevSettingsUserDefaultsDataSource, uses NSUserDefaults and is backwards compatible with the older implementation, so **no workflows or dependent code will break, and old saved settings will persist.**

The RCTDevMenu interface has not changed and is therefore also backwards-compatible, though
some methods are now deprecated.

In order to ensure that RCTDevSettings
Closes https://github.com/facebook/react-native/pull/11613

Reviewed By: mmmulani

Differential Revision: D4571773

Pulled By: javache

fbshipit-source-id: 25555d0a6eaa81f694343e079ed02439e5845fbc

Libraries/WebSocket/RCTWebSocketExecutor.m

@@ -56,6 +56,7 @@ RCT_EXPORT_MODULE()
   if (!_url) {
     NSUserDefaults *standardDefaults = [NSUserDefaults standardUserDefaults];
 
+    // TODO t16297016: this seems to be unused, remove?
     NSInteger port = [standardDefaults integerForKey:@"websocket-executor-port"];
     if (!port) {
       port = [[[_bridge bundleURL] port] integerValue] ?: 8081;