fix(types): improve SetupContext types

[antfu]

No description provided.

[jacekkarczmarczyk]

shouldn't slots, listeners and refs be using [keyin string]?: type?

With [key: string] for example slots.foo is always defined, but imho user should actually check if there's a slot with provided name

https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgGIHt3IN4ChnIDaA1hAJ4BcyAzmFKAOYC6Vt9IDuAvrrmGQAcUAIThRkAXhz4ipMslA06jJgH5Wyjt14wAriARhg6EMki1kAChiYqGdABpkAIzFVRUAJTSCCE9XQAGwgAOkD0BmtMEJhgAC84510EsIgOMAALJ1coGPjE5LjU9IzPbSA

[antfu]

Agree. Fixed, thanks!