Related to #799
With tabbing mode disabled, fullscreen (native) windows when creating a
new tab creates a window in the tab bar, but ALSO puts the window into a
new "screen" on macOS. When you click the tab bar, macOS animates
between screens. Its jarring!
This commit makes it so that in fullscreen we go back to automatic
tabbing even for new windows, which produces new windows within a tab.
This is normal default behavior for macOS tabbed programs. When you are
not in fullscreen or exit fullscreen, then the tabbing mode returns to
disabled so Ghostty can manage it.
