PlatformColors: add missing clearColor for iOS

[Simek]

Summary

This small PR adds missing clearColor to the avaiable PlatformColors on iOS.

Please let me know if you would like to see "Fixed colors" added to the PlatformColors. I can address this within this PR or create a separate one.

Changelog

[iOS] [Added] - PlatformColors: add missing clearColor

Test Plan

(I had to disable the Dark Mode to fix the RNTester readability problems)

Transparent/clear color has been added to the RNTester PlatformColors API example:

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: 2c896d3

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	7,371,273	76
android	hermes	armeabi-v7a	6,998,341	72
android	hermes	x86	7,811,769	72
android	hermes	x86_64	7,703,311	68
android	jsc	arm64-v8a	9,516,326	84
android	jsc	armeabi-v7a	9,135,538	80
android	jsc	x86	9,382,406	80
android	jsc	x86_64	9,963,324	80

Base commit: 2c896d3

[facebook-github-bot]

@sammy-SC has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @Simek in b7167c2.

^{When will my fix make it into a release? | Upcoming Releases}