[v3] Implement new Concept Exercise: extensions

[wneumann]

This issue describes how to implement the extensions concept exercise for the Swift track.

Getting started

Please please please read the docs before starting. Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before diving into the implementation, please read the following documents:

• The features of v3.
• Rationale for v3.
• What are concept exercises and how are they structured?

Please also watch the following video:

• The Anatomy of a Concept Exercise.

Goal

This concept exercise should convey a basic understanding of how to write extensions in Swift.

Things to teach

• Extension syntax
• Adding computed properties
• Adding initializers and methods
• Adding types

Things not to teach

• subscripts
• protocol conformance
• restricting extensions to specific types

Concepts

• extensions

Prerequisites

• structs-and-classes
• properties
• medhods
• initialization

Resources to refer to

Hints

• Extension syntax
• Adding computed properties
• Adding initializers
• Adding methods
• Adding nested types

After

• Extension syntax
• Adding computed properties
• Adding initializers
• Adding methods
• Adding nested types

Representer

TBD

Analyzer

TBD

Implementing

See the Swift track implementation guide for guidance.

Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.