[v3] Implement new Concept Exercise: extensions

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][docs-features-of-v3].
- [Rationale for v3][docs-rationale-for-v3].
- [What are concept exercises and how are they structured?][docs-concept-exercises]

Please also watch the following video:

- [The Anatomy of a Concept Exercise][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][extension-syntax]
- [Adding computed properties][extensions-properties]
- [Adding initializers][extensions-init]
- [Adding methods][extensions-methods]
- [Adding nested types][extensions-types]

### After

- [Extension syntax][extension-syntax]
- [Adding computed properties][extensions-properties]
- [Adding initializers][extensions-init]
- [Adding methods][extensions-methods]
- [Adding nested types][extensions-types]

## Representer

TBD

## Analyzer

TBD

## Implementing

See [the Swift track implementation guide][swift-implementing-concept-exercises] for guidance.

## Help

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

[swift-implementing-concept-exercises]: https://github.com/wneumann/v3/blob/master/languages/swift/reference/implementing-a-concept-exercise.md
[extension-syntax]: https://docs.swift.org/swift-book/LanguageGuide/Extensions.html#ID470
[extensions-properties]: https://docs.swift.org/swift-book/LanguageGuide/Extensions.html#ID152
[extensions-init]: https://docs.swift.org/swift-book/LanguageGuide/Extensions.html#ID153
[extensions-methods]: https://docs.swift.org/swift-book/LanguageGuide/Extensions.html#ID154
[extensions-types]: https://docs.swift.org/swift-book/LanguageGuide/Extensions.html#ID157
[docs-concept-exercises]: ../../../docs/concept-exercises.md
[docs-rationale-for-v3]: ../../../docs/rationale-for-v3.md
[docs-features-of-v3]: ../../../docs/features-of-v3.md
[anatomy-of-a-concept-exercise]: https://www.youtube.com/watch?v=gkbBqd7hPrA