Allow type augmentation

As of now, for me, v3.1.4 (latest) is not working, gives me some compilation errors, so I using these versions:
```
"@reactour/tour": "2.10.3",
"resolutions": {
"@reactour/utils": "0.3.0",
"@reactour/popover": "0.4.1",
"@reactour/mask": "0.5.1"
}
```

My application is context aware, so I need a key property on `StepType` interface.
I tried to augment types by creating `@reactour.d.ts`
Here is its contents:
```
import { StepType as _StepType } from '@reactour/tour';

declare module '@reactour/tour' {
type StepType = _StepType & {
key: string;
}
}
```

Wherever libary uses StepType interface, I expect it to have `key` property now, but that's not the case.
e.g.
```
const { steps } = useTour();
steps[0].key <---- throws an error: Property 'key' does not exist on type 'StepType'
```

So, I guess, typings of this library has some issues. It will be awesome if it is fixed in a near future release.

And thanks for a great library!