fix(react-native-host): add support for bridgeless mode

[tido64]

Description

Bridgeless mode can now be enabled by setting the environment variable USE_BRIDGELESS=1. This build flag will enable bridgeless bits, but you can still disable it at runtime by implementing RNXHostConfig.isBridgelessEnabled.

See the full announcement here: https://reactnative.dev/blog/2023/12/06/0.73-debugging-improvements-stable-symlinks#new-architecture-updates

Test plan

1. If necessary, clone react-native-test-app, then link in react-native-host

   cd ../react-native-test-app
   git checkout tido/bridgeless
   yarn clean
   yarn
   yarn link -r ../rnx-kit/packages/react-native-host
   cd example
   

2. In a separate terminal, start Metro server and keep it running for the rest of the session

   yarn start
   

3. Test old architecture

   pod install --project-directory=ios
   yarn ios
   

4. Test new architecture
   • In ios/Podfile, change :fabric_enabled to true
   • Repeat commands in step 2
   • Verify that "Fabric" is on
5. Test new architecture + bridgeless
   • In ios/Podfile, change :bridgeless_enabled to true
   • Repeat commands in step 2
   • Verify that "Bridgeless" is on
6. Test Hermes
   • In ios/Podfile, change :hermes_enabled to true
   • Repeat commands in step 2
   • Verify that "Hermes" is on

Screenshots

New Architecture	+ Bridgeless	+ Hermes

[kelset]

Testing iOS:

old arch	fabric	bridgeless	hermes

the only weird thing about it was safe area view error, but nothing worth bringing up I guess.

---

Testing Android:

old arch	fabric	bridgeless

[kelset]

tested and it works (see comment above), changes LGTM