MergeSort[cpp]

#### This issue is about [MergeSort cpp]

- [x] Issue Title is in form `Algo/DS Name [Language]`.
- [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
- [x] This Algo/DS is worth adding to this repository.
- [x] Assign this issue to me (I want to work on this).
- [x] I ensure that I am not already assigned to 2 or more issues.
- [x] I will send my PR only after I'm assigned to this issue by any maintainer.
- [x] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

Start by browsing the repository's existing algorithm and C++ implementations to find the expected location and format for MergeSort. Add the missing MergeSort [cpp] implementation following those conventions, then verify it behaves correctly using the repository's available checks.

Written by the indexing model from the issue text.