feature(swift): Reimplement merge_sort and top_k

[nuomi1]

If this PR is related to coding or code translation, please fill out the checklist and paste the console outputs to the PR.

• I've tested the code and ensured the outputs are the same as the outputs of reference code.
• I've checked the code (formatting, comments, indentation, file header, etc) carefully.
• The code does not rely on a particular environment or IDE and can be executed on a standard system (Win, macOS, Ubuntu).

[nuomi1]

result of `top_k.swift`

最大的 3 个元素为
堆的数组表示：[3, 7, 6]
堆的树状表示：
    /——— 6
——— 3
    \——— 7

result of `merge_sort.swift`

归并排序完成后 nums = [0, 1, 2, 3, 4, 5, 6, 7]

[krahets]

Thanks for the PR!