fix(26058): add MergeTuple for Object.assign's return type

[hyden-tan]

Add MergeTuple type, it support convert tuple to intersection. So, Object.assign can use it to define return type better.

/**
* Return a empty object type if the param's type is undefined
*/
type Undefined2Object<T> = T extends undefined ? {} : T;

/**
* Remove the first two item of tuple
*/
type RemoveFirstTwo<T extends unknown[], First, Second> =  T extends [a: First, b: Second, ...rest: infer P] ? P : [];

/**
* Convert tuple to intersection with tuple item's type
*/
type MergeTuple<T extends unknown[]> = T[2] extends undefined
  ? Undefined2Object<T[0]> & Undefined2Object<T[1]>
  : MergeTuple<[T[0], T[1]]> & MergeTuple<RemoveFirstTwo<T, T[0], T[1]>>;

Fixes #
#26058

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