Add shortest path by BFS (#1870)

* Create breadth_first_search_shortest_path.py

* updating DIRECTORY.md

* Reduce side effect of `shortest_path`

For the sake of future testing and documentation -

* fixup! Format Python code with psf/black push

* Fix typo `separately`

* Change to get() from dictionary

Co-Authored-By: Christian Clauss <cclauss@me.com>

* Move graph to the top

* fixup! Format Python code with psf/black push

* Add doctest for shortest path

* Add doctest for BFS

* fixup! Format Python code with psf/black push

* Add typings for breadth_first_search_shortest_path

* fixup! Format Python code with psf/black push

* Remove assert from doctests

* Add blank line to doctest

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: John Law <johnlaw.po@gmail.com>
Co-authored-by: Christian Clauss <cclauss@me.com>
Co-authored-by: John Law <johnlaw@linux.com>