Example.swift

struct TitleTextStyle: TextStyle {
    func makeBody(
        _ configuration: TextStyleConfiguration
    ) -> some View {
        configuration.text
            .font(.title)
            .fontWeight(.bold)
            .foregroundColor(.accentColor)
    }
}

// ...

Text(title)
    .textStyle(TitleTextStyle())

TextStyle.swift

protocol TextStyle {
    associatedtype Body: View

    func makeBody(
        _ configuration: TextStyleConfiguration
    ) -> Body
}

struct TextStyleConfiguration {
    let text: Text
}

extension Text {
    func textStyle<S>(
        _ style: S
    ) -> some View where S: TextStyle {
        let configuration = TextStyleConfiguration(
            text: self
        )

        return style.makeBody(configuration)
    }
}