Merge and add benchmarks to palindrome algorithms in the strings/ directory (#8749)

* refactor: Merge and add benchmarks to palindrome

* updating DIRECTORY.md

* chore: Fix failing tests

* Update strings/palindrome.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update palindrome.py

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Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>