Add a configuration for macOS users to explicitly set the NSWindow's Appearance to Dark or Light.

[kennycarruthers]

It would be nice to have a configuration option that allows macOS users to explicitly set the appearance of the NSApp used by WezTerm rather than only supporting the current appearance of the system.

For example, I normally have the 'Light' appearance set at the system level, but would prefer to have WezTerm launch in the 'Dark' appearance. It's a minor improvement, but it means that the NSWindow's titlebar better matches the dark theme I'm likely using inside the terminal.

(Apple does something like this for their "pro" apps, where they always launch in Dark mode.)

Given there's already config.macos_window_background_blur, perhaps config.macos_application_appearance or config.macos_window_appearance could be considered.

Apple's documentation for setting the appearance on the application is here:

https://developer.apple.com/documentation/appkit/nsapplication/2967170-appearance?language=objc