GTK: ability to load custom CSS

[jcollie]

Add a config entry gtk-custom-css that, when set, will load custom CSS from a file. The setting can be repeated to load multiple files and files can be marked as optional by prefixing it with a ?. The custom CSS will be added as priority USER so it should take precedence over any CSS set by Ghostty or the system

[mitchellh]

#4089 Approved