Style Improvements for project_euler/problem_26 (#2958)

* add typehints and docstrings

* add typehint and default value

* add typehint and default value. Removed unused variable.

* do not modifiy the given solution

* add doctests

* update sol1 after running black

* add typehint, docstring, and doctest

* update sol2 after running black

* add full problem statement and solution function with typehint and doctest

* renamed original function instead of adding new one

* don't alter original solution