[Library] Apply letter spacing for fonts

[pylapp]

Context

Today we did not implement yet the letter spacing feature using the FontLetterSpacingRawToken tokens.
In fact, this feature was not implemented until now because we got unusable values from Figma.
Since the tokenator can update these tokens, we have now usable values.

However, we need confirmation about the SwiftUI API to use, i.e. kerning or tracking.
In fact, the behaviour is not the same: kerning tries to prevent ligatures and tracking may add leading and trailing whitespace.

Documentation for tracking: https://developer.apple.com/documentation/swiftui/text/tracking(_:)
Documentation for kerning: https://developer.apple.com/documentation/swiftui/text/kerning(_:)

See the difference in the screenshot:

For the Swift code:

#Preview {
    Text("(0.18) Tracking - Nous vivons tous dans le ventre d'un chien géant")
        .tracking(0.18)
        .border(.green)

    Text("(0.18) Kerning - Nous vivons tous dans le ventre d'un chien géant")
        .kerning(0.18)
        .border(.green)

    Text("(-1.08) Tracking - Nous vivons tous dans le ventre d'un chien géant")
        .tracking(-1.08)
        .border(.blue)

    Text("(-1.08) Kerning - Nous vivons tous dans le ventre d'un chien géant")
        .kerning(-1.08)
        .border(.blue)
    
    Text("(10) Tracking - Nous vivons tous dans le ventre d'un chien géant")
        .tracking(10)
        .border(.red)

    Text("(10) Kerning - Nous vivons tous dans le ventre d'un chien géant")
        .kerning(10)
        .border(.red)
}

Definition of Ready

• Got answer from @MaximeTonnerre to know if tracking or kerning must be used

Definition of Done

• Update TypographyModifier (beware, feature available for iOS16+, not iOS 15)
• Update UI tests references images
• Update documentation
• Update CHANGELOG

[pylapp]

tracking should be prefered