Length of Longest Common Subsequence [CPP]

#### This issue is about [Algo/DS Name](link to resource for the Algo/DS).

- [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.

The issue requests a C++ implementation of the Length of Longest Common Subsequence, but it names no file, test, or entry point. First inspect the repository's existing C++ algorithm and data-structure implementations and contribution conventions; done means the algorithm is added in the expected location and follows the repository's established format.

Written by the indexing model from the issue text.