Removing index signature from Typescript type definition for Context
I realize Koa is not a Typescript project, but I wanted to call out a potentially big change I'm proposing to the type definition over on DefinitelyTyped. DefinitelyTyped has a policy of automatically merging PRs that have no comments/objections, and I'd like to make sure this doesn't sneak in under the radar of anyone.

The full explanation can be found on [my DefinitelyTyped PR](https://github.com/DefinitelyTyped/DefinitelyTyped/pull/26008), but the tl;dr is:

Remove the index signature from the Context interface to enforce stricter type checking. You can add it back yourself if you prefer a looser approach, but going the other direction (i.e. removing the index signature in your own project to enforce stricter types) is not currently possible in Typescript.

This means that by default you won't be able to randomly assign properties to `ctx` anymore - you'd need to augment the Koa module with the property's type definition:

```ts
declare module 'koa' {
  export interface Context {
    myCustomProp: boolean;
  }
}
```

Or add the index signature to get back today's behavior:

```ts
declare module 'koa' {
  export interface Context {
    [key: string]: any;
  }
}
```