New terminal-notifier formula (first version, revision 3)

The `terminal-notifier` utility (analogous to Growl's `growlnotify` tool) offers a CLI for displaying Apple Notification Center messages.

To use the program as distributed, one must call the inner binary of an app bundle (due to a library-linking quirk the author notes, in https://github.com/alloy/terminal-notifier/blob/master/README.markdown) – this formula sidesteps this awkward mode of execution with an exec script (see L#15).

New terminal-notifier formula (with test code codoned off apropos its function, redundant calls removed, and adjusted syntax in respect of Homebrew code conventions)

Closes #18511.

Signed-off-by: Samuel John <github@SamuelJohn.de>