feat: add Expelliarmus spell

[VanshSahay]

Fixes

Fixes #15 by @jatinkrmalik

Description

Adds the Expelliarmus spell which kills a process given the PID

Technical details

Uses kill <pid> and has a few essential system processes excluded

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.