Implemented font-variation-settings for iOS

[davebcn87]

Summary:

This PR extends the support on React Native for variable fonts. This allow to customize any font setting: wght, slnt, wdth are common examples. A font designer can use any custom attribute, this is what we made this feature compatible with any property.

It follows the css standard font-variation-settings. This attribute is a string, formatted as a key value pairs separated by commas:

.boldText {
   font-variation-settings: "'whgt' 850";
}

Screenshots

Fabric	Paper

The Android implementation of this feature is being implemented by another PR from @matinzd. We've been pairing on this solution to follow a similar implementation for both platforms.

Changelog:

[iOS] [ADDED] - Add fontVariationSettings support for variable fonts in iOS

Test Plan:

I've added examples in the rn-tester app using variable fonts. I've tested the following scenarios:

• Common attributes: wght, wdth, slnt.
• Custom attributes: FILL
• Mixing font-variation-settings with other styles that affect font descriptors, like font-variant. See tabular-nums example.
• It works in both architectures, Fabric and Paper (See screenshot).

This is an example of the final implementation usage code:

<Text style={{fontFamily: 'Inter', fontVariationSettings: '"wght" 800'}}>Inter wght 800</Text>

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[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	19,542,634	+27
android	hermes	armeabi-v7a	n/a	--
android	hermes	x86	n/a	--
android	hermes	x86_64	n/a	--
android	jsc	arm64-v8a	22,896,376	+15
android	jsc	armeabi-v7a	n/a	--
android	jsc	x86	n/a	--
android	jsc	x86_64	n/a	--

Base commit: a908387
Branch: main