Add a solution for Project Euler 49 (#2702)

* added doctests in modular_exponential.py

* added doctests in modular_exponential.py

* added URL link

* updating DIRECTORY.md

* Add problem 49 solution

* updating DIRECTORY.md

* Fix several mistakes

These fixes are intended to follow the CONTRIBUTING.md

* Move the import statements lower

* Update project_euler/problem_49/sol1.py

Co-authored-by: Dhruv <dhruvmanila@gmail.com>

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Dhruv <dhruvmanila@gmail.com>