Classify .bash_functions as Shell

[Nixinova]

• Resolves add .bash_functions #6234

Description

Adds .bash_functions as a Shell filename.

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub:
    • Search results for each extension:
      • path:**/.bash_functions - 281 files. Assuming 1 per repo.
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • tlrh314/atlas-web
    • Sample license(s):
      • GNU 3
  • I have included a change to the heuristics to distinguish my language from others using the same extension.