Add new macOS View prop allowsVibrancy

[Saadnajmi]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

Appkit has a prop allowsVibrancy that will allow Appkit to subtly change the colors of an NSView inside an NSVisualEffectView to get that "vibrant" effect. There's a property we can override on NSView for custom NSViews so we can get the effect too. Let's expose it on macOS so that we can set it through Javascript. One caveat is the Appkit property is readonly, so I had to add a new RCTView property allowsVibrancyInternal that the RCTView getter then reads.

https://developer.apple.com/documentation/appkit/nsview/1483793-allowsvibrancy

Changelog

[MACOS] [ADDED] - Add new macOS View prop allowsVibrancy

Test Plan

Prototyping this inside FluentUI React Native (by just.. making every RCTView set allowsVibrancy = true), you can clearly see a difference between the two Callouts. Both have a solid background color set to them, but one will "blend" the colors of the window behind it, AKA, the vibrant effect.

Without (notice the solid pink):

With (notice the pink is blended on the top left):

[AdrianFahrbach]

Does this enable us to replicate translucent sidebars like this?

Because I'm currently using a NSVisualEffectView and using this with custom themes is quite difficult.

Oh and sorry for digging through your pull requests, I hope you don't mind 😅
I really enjoy using your library so far!

[Saadnajmi]

@AdrianFahrbach Yep! See this PR where I make a native module for VibrancyView: microsoft/fluentui-react-native#3026

I wanted to upstream it to https://github.com/Kureev/react-native-blur eventually, there's just more extra code needed than I'd like.