Add key features

_This issue is part of the migration to v3. You can read full details about the various changes [here](https://github.com/exercism/v3-launch/)._

In Exercism v3, each track must specify exactly six "key features". Exercism uses these features to highlight the most interesting, unique or "best" features of a language to a student.

Key features are specified in the top-level `"key_features"` field in the track's `config.json` file and are defined as an array of objects, as specified in [the spec](https://github.com/exercism/v3-docs/blob/main/anatomy/tracks/config-json.md#key-features).

## Goal

The `"key_features"` field in the `config.json` file should be updated to describe the six "key features" of this track. See [the spec](https://github.com/exercism/v3-docs/blob/main/anatomy/tracks/config-json.md#key-features).

## Example

```json
{
"key_features": [
{
"icon": "features-oop",
"title": "Modern",
"content": "C# is a modern, fast-evolving language."
},
{
"icon": "features-strongly-typed",
"title": "Cross-platform",
"content": "C# runs on almost any platform and chipset."
},
{
"icon": "features-functional",
"title": "Multi-paradigm",
"content": "C# is primarily an object-oriented language, but also has lots of functional features."
},
{
"icon": "features-lazy",
"title": "General purpose",
"content": "C# can be used for a wide variety of workloads, like websites, console applications, and even games."
},
{
"icon": "features-declarative",
"title": "Tooling",
"content": "C# has excellent tooling, with linting and advanced refactoring options built-in."
},
{
"icon": "features-generic",
"title": "Documentation",
"content": "Documentation is excellent and exhaustive, making it easy to get started with C#."
}
]
}
```

## Tracking

https://github.com/exercism/v3-launch/issues/5

Read the linked key-features specification and inspect the track's top-level config.json. Define six feature objects describing Shen, following the required fields and icon conventions in the spec; the work is done when the configuration contains exactly six compliant entries.

Written by the indexing model from the issue text.