[v3] Implement new Concept Exercise: filter and compactMap

This issue describes how to implement the filter 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 filter in Swift.

## Learning Objectives

- What is `filter`?
- Sequence.filter
- What is `compactMap`?

## Out of Scope

- Other higher-order functions

## Concepts

- `filter`

## Prerequisites

- `map`

## Resources to refer to

### Hints

- [Sequence.filter][sequence-filter-docs]
- [Sequence.compactMap][sequence-compactmap-docs]

### After

- [Sequence.filter][sequence-filter-docs]
- [Sequence.compactMap][sequence-compactmap-docs]

## 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
[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

[sequence-filter-docs]: https://developer.apple.com/documentation/swift/sequence/3018365-filter
[sequence-compactmap-docs]: https://developer.apple.com/documentation/swift/sequence/2950916-compactmap