This largely copies over Spicy's `.clang-format` configuration file. The
one place where we deviate is header include order since Zeek depends on
headers being included in a certain order.
This also includes:
- Deprecating the NetSessions name.
- Renaming the zeek::sessions global to zeek::session_mgr and deprecating the old name.
- Renaming Sessions.{h,cc} to SessionManager.{h,cc}.