Add tmux.conf and .tmux.conf as filenames for shell. There was already the extension .tmux but the default config filenames were missing.

Description

classify tmux.conf and .tmux.conf as shell too, so we can get the sweet syntax highlighting

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 filename:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.tmux.conf+bind
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*tmux.conf+bind
  • I have included a real-world usage sample for all extensions filenames added in this PR:
    • Sample source(s):
      • https://github.com/lildude/dotfiles/blob/main/chezmoi/dot_config/tmux/tmux.conf 😉
    • Sample license(s):
      • https://github.com/lildude/dotfiles/blob/main/LICENSE
  • I have included a change to the heuristics to distinguish my language from others using the same extension. (Not applicable, the filename shouldn't be universally used for anything else afaik)