RN: Add RootTag Type to TurboModule

Summary:
Adds `RootTag` as a valid type for arguments and return types in TurboModules (on both Android and iOS).

This will enable us to change `RootTag` into an opaque type. There are two compelling reasons to do this:

- JavaScript will no longer be able to safely depend on `RootTag` being a number (which means we can change this in the future).
- Call sites using `unstable_RootTagContext` will can get a `RootTag`, but call sites using the legacy `context.rootTag` will not. This means the opaque type will give us a strategy for migrating away from legacy context and eventually making `unstable_RootTagContext` the only way to access `RootTag`.

Changelog:
[Internal]

(Note: this ignores all push blocking failures!)

Reviewed By: RSNara

Differential Revision: D21127170

fbshipit-source-id: baec9d7ad17b2f8c4527f1a84f604fc0d28b97eb

Libraries/TurboModule/samples/NativeSampleTurboModule.js

@@ -10,7 +10,7 @@
 
 'use strict';
 
-import type {TurboModule} from '../RCTExport';
+import type {RootTag, TurboModule} from '../RCTExport';
 import * as TurboModuleRegistry from '../TurboModuleRegistry';
 
 export interface Spec extends TurboModule {
@@ -26,6 +26,7 @@ export interface Spec extends TurboModule {
   +getString: (arg: string) => string;
   +getArray: (arg: Array<any>) => Array<any>;
   +getObject: (arg: Object) => Object;
+  +getRootTag: (arg: RootTag) => RootTag;
   +getValue: (x: number, y: string, z: Object) => Object;
   +getValueWithCallback: (callback: (value: string) => void) => void;
   +getValueWithPromise: (error: boolean) => Promise<string>;