Support typed configuration via an `ava.config.ts` file

It'd be useful if we could use TypeScript to define our Ava configurations via an `ava.config.ts` file.

Ideally, we'd be able to structure them like so:

```ts
// Importable `Configuration` type
import { Configuration } from 'ava';

// Typed configuration object
const configuration: Configuration = {
// ... config goes here
};

export default configuration;
```

Webpack supports similar functionality with `webpack.config.ts` files (although I believe `ts-node` is required for those).

The issue names ava.config.ts and compares it with webpack.config.ts, but no repository files or tests are identified. Start by locating the existing AVA configuration-loading entry point and TypeScript configuration declarations, then check how ts-node is handled; done means a typed ava.config.ts can be loaded and the behavior is covered by tests.

Written by the indexing model from the issue text.