Add Swift Package Manager

[Lancelotbronner]

I would like to use this in some of my projects, is it possible to implement support to the Swift Package Manager by adding a Package.swift?

[Lancelotbronner]

Something like that:

// swift-tools-version:5.1
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "StyledTextKit",
    products: [
        .library(name: "StyledTextKit", targets: ["StyledTextKit"]),
    ],
    targets: [
        .target(name: "StyledTextKit", path: "Source"),
    ]
)