typescript mixin example does not compile

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I get an ts error with the example from https://lit.dev/docs/composition/mixins/#typing-the-subclass (second example - When a mixin adds new public/protected API)

The errors reads:
```
Conversion of type '{ new (...args: any[]): UploadMixinClass; prototype: MyMixin.UploadMixinClass; } & T' to type 'Constructor & T' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
Type '{ new (...args: any[]): UploadMixinClass; prototype: MyMixin.UploadMixinClass; } & T' is not comparable to type 'Constructor'.
Type 'UploadMixinClass & PolymerElement' is not comparable to type 'UploadMixinInterface'.
Property 'renderHighlight' is protected but type 'UploadMixinClass' is not a class derived from 'UploadMixinInterface'.ts(2352)
```

I think we should have this instead (adding `as unknown` on the return), to help ts beginners like myself:
``` ts
type Constructor = new (...args: any[]) => T;

export declare class UploadMixinInterface {
highlight: boolean;
protected renderHighlight(): unknown;
}

export const MyMixin = >(superClass: T) => {
class UploadMixinClass extends superClass {
@property() highlight = false;
protected renderHighlight() {
/* ... */
}
};
// Cast return type to your mixin's interface intersected with the superClass type
return UploadMixinClass as unknown as Constructor & T;
}
```

Open the linked Lit documentation page and inspect the second example in “When a mixin adds new public/protected API.” Reproduce the reported TypeScript error, update the example’s return cast as proposed, and verify that the example compiles without the reported error.

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