Add .ignore as ignore list filename

[spenserblack]

Description

Surprisingly, this generic filename wasn't yet added 😆 It's been gaining some traction, being used for popular tools like scc and ripgrep

Checklist:

• I am adding a new extension filename to a language.
  • The new extension filename is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?q=path%3A%2F%28%5E%7C%5C%2F%29%5C.ignore%24%2F+NOT+is%3Afork&type=code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • This was manually written, given the simplicity of ignore lists