feat: only resolve "strip()" for schema when used as an object field

[jquense]

Schema with strip() resolve to never types so that they are removed from the object type, but this also meant they resolved to never even outside of an object which isn't accurate since strip only has an effect for object properties. This also meant that Schema types were harder to use b/c never is like the anti any and always resulted in schema that were incompatible with each other even if they were.

string().strip().cast() // string | undefined

object({
   unused: string().strip()
}).cast()  // {}

Now the strip flag is only affects the output type when used in an object shape, which means that you can do things like the following without issue:

export function yup<T>(schema: AnySchema<T>) {
  return (value: unknown) => schema.validateSync(value);
}

yup(string()); 
yup(string().default('')); 

fixes #1974