[RN][macos] Enable Hermes for RNTester

Summary:
**Context**

On Core RN, Hermes for iOS can be enabled by setting a flag in the Podfile
https://reactnative.dev/docs/hermes#ios

| Since React Native 0.64, Hermes also runs on iOS. To enable Hermes for iOS, edit your ios/Podfile file and make the change illustrated below:
```
   use_react_native!(
     :path => config[:reactNativePath],
     # to enable hermes on iOS, change `false` to `true` and then install pods
     # By default, Hermes is disabled on Old Architecture, and enabled on New Architecture.
     # You can enable/disable it manually by replacing `flags[:hermes_enabled]` with `true` or `false`.
     :hermes_enabled => true
   )
```
In the RNTester Podfile, Hermes is enabled using envvar:
https://github.com/facebook/react-native/blob/main/packages/rn-tester/Podfile#L27
```
  # Hermes is now enabled by default.
  # The following line will only disable Hermes if the USE_HERMES envvar is SET to a value other than 1 (e.g. USE_HERMES=0).
  hermes_enabled = !ENV.has_key?('USE_HERMES') || ENV['USE_HERMES'] == '1'
```
Build command: `USE_HERMES=1 bundle exec pod install`

This will install the Hermes runtime Pod (not build it from scratch) & thus enable the `RCT_USE_HERMES` macro.

https://www.internalfb.com/code/fbsource/[9f57823a75a40d3f8559c8f1b7ae0add8e95d6dc]/xplat/js/react-native-github/packages/rn-tester/RNTester/AppDelegate.mm?lines=10-16

---

The documentation for enabling Hermes on RN Desktop macOS are outdated:
https://microsoft.github.io/react-native-windows/docs/hermes#hermes-on-macos

> Install the npm package yarn add 'hermes-engine-darwin@^0.4.3'

* `hermes-engine-darwin` is no longer required

> Add (or uncomment) the following pod dependencies to your macOS target in your Podfile:
pod 'React-Core/Hermes', :path => '../node_modules/react-native-macos/'
pod 'hermes', :path => '../node_modules/hermes-engine-darwin'
pod 'libevent', :podspec => '../node_modules/react-native-macos/third-party-podspecs/libevent.podspec'

* Setting `USE_HERMES=1` during `pod install= replaces all of this

> Copy
Run pod install
Be sure to set your target's deployment target to at least 10.14 before running pod install

* `USE_HERMES=1 bundle exec pod install --verbose`

---

On RN Desktop, the Hermes flag was [set to false](microsoft#780) due to M1 build reasons which have since been resolved.
- microsoft#952
- microsoft#781

Curiously, the `RNTester-macOS` target AppDelegate was never updated to import & use Hermes when  `RCT_USE_HERMES` was `true`. Only the `RNTester` for mobile had the correct Hermes usage.

**RNTester-macOS:** https://github.com/microsoft/react-native-macos/blob/main/packages/rn-tester/RNTester-macOS/AppDelegate.mm

**RNTester:** https://github.com/microsoft/react-native-macos/blob/main/packages/rn-tester/RNTester/AppDelegate.mm

**Change**

* Remove `pods(:hermes_enabled => true)` in favor of passing `USE_HERMES=1` to `pod install` (This is how it's done on RNTester iOS)
* Copy Hermes support to `RNTester-macOS` AppDelegate

Test Plan: **Install from scratch**

Differential Revision: https://phabricator.intern.facebook.com/D38277077

packages/rn-tester/Podfile

@@ -86,7 +86,7 @@ end
 
 target 'RNTester-macOS' do
   platform :osx, '10.15'
-  pods(:hermes_enabled => false)
+  pods()
 end
 
 target 'RNTester-macOSUnitTests' do

packages/rn-tester/RNTester-macOS/AppDelegate.mm

@@ -8,8 +8,21 @@
 
 #import "AppDelegate.h"
 
-#import <React/CoreModulesPlugins.h>
+#ifndef RCT_USE_HERMES
+#if __has_include(<reacthermes/HermesExecutorFactory.h>)
+#define RCT_USE_HERMES 1
+#else
+#define RCT_USE_HERMES 0
+#endif
+#endif
+
+#if RCT_USE_HERMES
+#import <reacthermes/HermesExecutorFactory.h>
+#else
 #import <React/JSCExecutorFactory.h>
+#endif
+
+#import <React/CoreModulesPlugins.h>
 #import <React/RCTBridge.h>
 #import <React/RCTBundleURLProvider.h>
 #import <React/RCTCxxBridgeDelegate.h>
@@ -106,7 +119,11 @@ - (NSURL *)sourceURLForBridge:(__unused RCTBridge *)bridge
 #endif
 
   __weak __typeof(self) weakSelf = self;
+#if RCT_USE_HERMES
+  return std::make_unique<facebook::react::HermesExecutorFactory>(
+#else
   return std::make_unique<facebook::react::JSCExecutorFactory>(
+#endif
       facebook::react::RCTJSIExecutorRuntimeInstaller([weakSelf, bridge](facebook::jsi::Runtime &runtime) {
         if (!bridge) {
           return;