According to the GNOME human interface guidelines, buttons for the main
user actions, such as new, add, open, and back should be placed at the
start of the header bar. (https://developer.gnome.org/hig/patterns/containers/header-bars.html#header-bar-buttons)
Moving the new tab button to the start of the header bar brings Ghostty
in line with other GNOME applications such as gedit and gnome-terminal.