Use system API on macOS to open links

[sindresorhus]

Currently, it uses the open command. The problem with using that is that it makes it impossible for browser pickers apps to detect that the link is opened from Ghostty, because by the time the app checks for the source app, the open command has exited. It should also be slightly faster to use the system API: https://developer.apple.com/documentation/appkit/nsworkspace/open(_:)?language=objc

I'm referring to: https://github.com/ghostty-org/ghostty/blob/72d085525b22d66468c5969a4d507a0fa68d4a04/src/os/open.zig#L32C9-L41C11

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Specifically, Velja uses the source app to let users create advanced rules for which browser to use depending on the source app.

[mitchellh]

Approved #5256