When the OS (as the default browser), "open -a iTerm2 <url>", Velja, or
vim's gx hands iTerm2 an http/https/file URL via the GetURL Apple event,
open it in the built-in web browser instead of launching a terminal,
unless the user has bound the scheme to a specific profile. If the
browser plugin isn't installed, offer it once (silenceable) and fall
back to the legacy behavior.