docs: Add Swift/macOS tutorial

[felixrieseberg]

Description of Change

This is my fourth PR in a series of tutorials on how to integrate native code with Electron. All the maintainers are intimately familiar with Electron's "just write native code" superpower, but very little of that is prominently documented on the homepage. To make this easy to review, I'll be making a series of PRs.

This third tutorial covers building a Node.js addon with Objective-C/Swift/SwiftUI targeting macOS. It's a meaty one because I'm covering how to build native GUI using SwiftUI.

• Native Code and Electron ✅
• Native Code and Electron: C++ (Windows) ✅
• Native Code and Electron: Objective-C (macOS) (In progress, docs: Add ObjC/macOS tutorial #46694)
• Native Code and Electron: Swift (macOS) ← You are here
• Native Code and Electron: C++ (Linux)
• Native Code and Electron: Rust

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation, tutorials, templates and examples are changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: no-notes