allow InputProps and OutputProps to be typed in generated Load

[ivanhofer]

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Same fix as here: #4160
But this time for the generated Load type.

I also have restricted all the Params and Props types with the extends keyword.
Maybe it makes sense to create a "base type" and use it everywhere:

+type BaseParams = Record<string, string>
+type BaseProps = Record<string, any>

export interface Load<
-	Params extends Record<string, string> = Record<string, string>,
-	InputProps extends Record<string, any> = Record<string, any>,
-	OutputProps extends Record<string, any> = InputProps
+	Params extends BaseParams = BaseParams,
+	InputProps extends BaseProps = BaseProps,
+	OutputProps extends BaseProps = InputProps
> {
	(input: LoadInput<Params, InputProps>): MaybePromise<
		Either<Fallthrough, LoadOutput<OutputProps>>
	>;
}

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Latest commit: 2d367e7

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[ivanhofer]

I also have noticed the same issue with the generated RequestHandler type.
It was already included in the docs but the generated type was wrong. I included the fix in this PR.

I'm not sure what to do with the Body type from the "private" types. Can I move it to index.d.ts?

[Rich-Harris]

Thanks! Yeah, it makes sense to expose Body. I've updated this PR so that it's exposed as ResponseBody