feat: Add alpha, prominence, and contentHeadroom settings for PlatformColor

[EvanBacon]

Summary:

In Expo, we encourage the use of PlatformColor since it gives users the most Apple-themed outcome on iOS. We've found LLMs often try to augment the opacity however. This PR adds support for such a modification.

In SwiftUI it's common to use tint properties on controls which results in a layered platform color that we can't easily replicate without prominence.

The text could be defined with PlatformColor("systemGreen"), but the backing layer can't be easily replicated. Now you can set the prominence on the color to replicate the style:

<View style={{ backgroundColor: PlatformColor({ name: 'systemGreen', prominence: "tertiary" }) }}>
  <Text style={{ color: PlatformColor('systemGreen') }}>
    Action
  </Text>
</View>

The API is clearly more clunky than SwiftUI .tint(.green) but user-space styling libraries such as react-native-css could implement as a tailwind class such as text-sf-green-3 ({ prominence: 'tertiary' }), text-sf-red/50 (50% opacity).

Since I was in there already, I also added the other UIColor modification applyingContentHeadroom (iOS 26).

Alternative APIs:

• Tried a builder pattern PlatformColor("systemMint").alpha(0.5) — but the types were less sensible and chaining wouldn't work.
• PlatformColor("systemMint", {alpha: 0.5}) — Perform type check internally to disable color fallback behavior.
• PlatformColor("systemMint").withAlpha(0.5) — more robust but more sensible to add types for since PlatformColor('...').alpha(1).alpha(1) won't work.

Open to ideas for how we might implement alpha/prominence/content headroom on Android. It appears that if we modify the alpha factor then the color will no longer be dynamic/adaptive to system themes.

Changelog:

[IOS] [ADDED] - Added support for alpha, prominence, and contentHeadroom for Apple PlatformColors.

[ANDROID] [ADDED] - Added support for alpha PlatformColor on Android.

Test Plan:

• Updated RNTester