macos: support configuration via CLI arguments

This makes it so `zig build run` can take arguments such as
`--config-default-files=false` or any other configuration. Previously,
it only accepted commands such as `+version`.

Incidentally, this also makes it so that the app in general can now take
configuration arguments via the CLI if it is launched as a new instance
via `open`. For example:

    open -n Ghostty.app --args --config-default-files=false

This previously didn't work. This is kind of cool.

To make this work, the libghostty C API was modified so that
initialization requires the CLI args, and there is a new C API to try to
execute an action if it was set.

macos/Sources/Ghostty/Package.swift

@@ -61,9 +61,12 @@ extension Ghostty {
     /// its up to the env var being set in the correct circumstance.
     static var launchSource: LaunchSource {
         guard let envValue = ProcessInfo.processInfo.environment["GHOSTTY_MAC_LAUNCH_SOURCE"] else {
-            return .app
+            // We default to the CLI because the app bundle always sets the
+            // source. If its unset we assume we're in a CLI environment.
+            return .cli
         }
         
+        // If the env var is set but its unknown then we default back to the app.
         return LaunchSource(rawValue: envValue) ?? .app
     }
 }