Fabric iOS support

[fortmarek]

Description

Related: #196

This is an initial draft for Fabric support. I have hit a couple of issues we'll need to resolve, some of them are not a flash-list issue specifically:

• @react-navigation/stack seems not to be compatible with Fabric (I get the same error as here)
  • for now, I moved to @react-navigation/native-stack which seems to work and is not an issue (fixture-specific only)
• the hierarchy of views have changed, we'll need to change accessing AutoLayoutView subviews
• FlashListPerformanceView is not migrated to Fabric
• onBlankAreaEvent is currently not called in Fabric
• migrate reanimated to v3
• only elements during the first render are loaded - when scrolling, new ones are not added. This is possibly issue in recyclerlistview (possibly some assumption is no longer valid on the new arch)
• it is not possible to touch elements wrapped inside a Fabric native view (reproduced in the RN tester app here)

Minor issues (don't need to be resolved):

• Switching between Fabric and non-Fabric is confusing. You will get build errors because of the code generated by Fabric which might not be compatible with your pre-Fabric app.

Resolved issues:

• Some dependencies (listed when running USE_FABRIC=1 RCT_NEW_ARCH_ENABLED=1 bundle exec pod install) in react_native_pods turned on Fabric must have modular_headers: true when using Swift code
  • If you add that setting for relevant dependencies, the modular headers for React-RCTFabric and React-Core conflict
  • both of these will need to be fixed in react-native directly (it seems Swift support has not been properly tested)
  • both fixed here
• reanimated does not support Fabric which causes issues for our fixture app (Reminders example). It should be ready along with 0.69 release of react-native Update: It does now 🎉

I will continue working on those we can tackle and create appropriate issues or PRs (if possible) for those that are outside are repos.

Resources used during the migration:

• https://reactnative.dev/docs/next/new-architecture-library-ios (and other parts of the new architecture documentation)
• https://react-native-community.github.io/upgrade-helper/?from=0.67.4&to=0.68.1
• Commit bringing support for Fabric inside react-native-gesture-handler. The great thing here is that the library support both Fabric and Paper whereas the official documentation assumes you will want to support Fabric only.
• Example app showcasing migrating to Fabric along with a custom Fabric native component
• feat: support Fabric software-mansion/react-native-svg#1754

Reviewers’ hat-rack 🎩

• [ ]

Screenshots or videos (if needed)

Checklist

• I have added a changelog entry following the Keep a Changelog guidelines.

[fortmarek]

Closing in favor of #549