Add `site_bin_dir` counterpart for `user_bin_dir`

[PhrozenByte]

The user_bin_dir method currently lacks a site_bin_dir counterpart.

Some suggestions:

• Unix:
  • /usr/bin?
  • Or a /usr/local/bin:/usr/bin multipath? However, it was chosen not to use a multipath for site_log_dir (Add site_state_dir() counterpart for user_state_dir() #214), so platformdirs could choose to only support /usr/bin for consistency.
  • Or maybe even use $PATH for a multipath? However, the same inconsistency issue applies, additionally platformdirs would need to filter $PATH to exclude e.g. $HOME/.local/bin… Thus I don't think that this is really feasible.
• Android: user_bin_dir alias? As with any other site_*_dir counterpart…
• Windows:
  • %PROGRAMFILES%\$appauthor\$appname?
  • Or site_data_dir with an opinionated Bin subdirectory? But I'd say that this would be rather unexpected…
  • Or C:\Windows\System32? It's the only path that is in %PATH% by default, but that would be kinda crazy IMHO…
• Mac OS: /usr/local/bin? AFAIK one can't use /usr/bin on OS X, but I don't have a Mac…

Note that user_bin_dir currently doesn't accept $appauthor, $appname, $version.

If implemented, user_bin_dir should support the use_site_for_root option and redirect calls to site_bin_dir if enabled.