[Post-7.0] Improve readability of `Array.from` / `TypedArray.from` `mapFn` parameters

Currently, the mapFn argument of Array.from() is typed as (v: T, k: number) => U.
However, when this information is displayed in an IDE tooltip, it is difficult to grasp what values are passed to v and k.

To improve clarity, I propose changing the type definition to (element: T, index: number) => U, following the description of the same method in MDN Web Docs. The same applies to TypedArray.from().

Supplementary Information:

• The target file is internal/bundled/libs/lib.es2015.iterable.d.ts#L107.
• These changes are not intended to fix differences between 6.0 and 7.0, and according to CONTRIBUTING.md, the Pull Request should be submitted after the 7.0 release. I plan to submit a Pull Request for this once 7.0 is released.

internal/bundled/libs/lib.es2015.iterable.d.ts#L107 から始めて、Array.from と TypedArray.from の mapFn 宣言を見つけます。パラメーター名をリンク先の MDN の説明と比較し、両方の宣言が型や動作を変更せずに、要素とインデックスに説明的な名前を使用していることを確認します。