Add macOS app icon (#2723)

## Description

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Description and motivation for this PR.

Include 'Fixes #<number>' if this is fixing some issue.
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This PR simply adds app icon for `MacOSExample` app.  

## Test plan

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Describe how did you test this change here.
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The icon is visible as intended:

- from Xcode

![xcode_icon](https://github.com/software-mansion/react-native-gesture-handler/assets/56112901/2539aca9-47e8-4aad-8998-8f16c9375aa9)
- and from dock

![dock_icon](https://github.com/software-mansion/react-native-gesture-handler/assets/56112901/288e76bf-7bfb-4ad8-869f-d4529cde3d88)

Author: jfedak