Added possibility of separate text styles extension generation

[ttoxa-xa]

Description

In some cases we need to use exported separately from Label, for example - in text field etc.

Changes

Added an optional iOS param labelStyleSwift
Added generation of LabelStyle extensions with factory methods for each style
Make LabelStyle public
Updated CONFIG.md

[subdan]

1. Fix unit-tests.
2. Rename LabelStyle+Extension.swift to LabelStyle+extension.swift for consistency.
3. Fix code duplication. You create LabelStyle for the same text style two times:

here:

public extension LabelStyle {
    
    static func body() -> LabelStyle {
        LabelStyle(
            font: UIFont.body(),
            fontMetrics: UIFontMetrics(forTextStyle: .body),
            lineHeight: 24.0
        )
    }
    ...

and here:

public final class BodyLabel: Label {

    override var style: LabelStyle? {
        LabelStyle(
            font: UIFont.body(),
            fontMetrics: UIFontMetrics(forTextStyle: .body),
            lineHeight: 24.0
        )
    }
}

For each *Label class just call static method of LabelStyle extension.

public final class BodyLabel: Label {

    override var style: LabelStyle? {
        .body()
    }
}