Switch to using the existing UTType.unixExecutable constant for this
operator, which also lets us remove a failure path. Also, use the
completion-based setDefaultApplication() variant to handle errors.
This simplifies the code enough that we don't need the additional
NSWorkspace+Ghostty extension functions.
Fixes#5256
This updates the macOS apprt to implement the `OPEN_URL` apprt action to
use the NSWorkspace APIs instead of the `open` command line utility.
As part of this, we removed the `ghostty_config_open` libghostty API and
instead introduced a new `ghostty_config_open_path` API that returns the
path to open, and then we use the `NSWorkspace` APIs to open it (same
function as the `OPEN_URL` action).
