Knuth–Morris–Pratt [C++]

#### This issue is about [Knuth–Morris–Pratt](https://en.wikipedia.org/wiki/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm).

- [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.

No file, test, or entry point is named. Start by inspecting existing algorithm implementations in the repository and their C++ conventions, then add Knuth–Morris–Pratt in C++; done means the algorithm is included consistently with the repository’s other implementations.

Written by the indexing model from the issue text.