Fabric support on iOS

[j-piasecki]

Description

This PR adds Fabric integration to the FlashList on iOS. Here are some more important highlights:

• I've merged CellContainer with CellContainerComponentView as keeping a CellContainer in every CellContainerComponentView didn't look like a very good idea performance-wise
• In case of AutoLayoutView, I stayed with wrapping it with the Fabric Component (AutoLayoutViewComponentView). I didn't want to accidentally break the functionality converting in to obj-c. It may be possible that they can be merged via inheritance but I'm not that knowledgeable in interop between swift and obj-c and couldn't get it to work
• Event dispatchers on Fabric are auto generated so I added a field to AutoLayoutView containing a closure, which in turn is set by the AutoLayoutViewComponentView. This way the code in swift has access to the generated obj-c++ method.
• Due to the view flattening and the fact that the AutoLayoutView needs to be wrapped with respective ComponentView, the view hierarchy is different on Fabric. I added a getSubviews() method that has a different implementation when building for the new architecture.
• I've added a separate fabricfixture app to test Fabric integration without changing the original one.

I have tested the TwitterBenchmark and the performance there didn't seem to differ between architectures.
Fabric:

Device	Avg FPS	Min FPS	Max FPS	Max blank area	Cumulative blank area
iPhone 13 mini	60.1	59.9	60.9	0	0
iPhone 6s	59.1	54.9	60.3	0	0

Paper:

Device	Avg FPS	Min FPS	Max FPS	Max blank area	Cumulative blank area
iPhone 13 mini	60.1	59.9	60.6	0	0
iPhone 6s	58.1	54.6	60	0	0

The fabricfixture app is pretty basic at the moment (only TwitterBenchmark and commented out Paginated list) due to the compatibility of other libraries.

Reviewers’ hat-rack 🎩

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Screenshots or videos (if needed)

Checklist

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

[j-piasecki]

Closed in favor of #550.