This updates the Ghostty icon to be compatible with macOS Tahoe
(supports glass effects, light/dark, tinting, etc.). This icon is made
in the new Apple Icon Composer as the source format, and all other
formats are exported from it.

This commit also updates the icon for non-Apple platforms because the
icon is fundamentally the same and I don't see any reason to maintain
multiple icons of fundamentally the same design and style.

This commit also includes updates to the macOS app so that the About
Window and so on will use the new icon.