add WguiFontSystem, remove FONT_SYSTEM singleton, custom fonts, add Light font weight

there are a few gzip-compressed ttf as for now, looks like variable fonts aren't parsed properly by cosmic_text. Not sure why. Also, we probably need to have a fallback for CJK characters in the future, or just fallback to the built-in ones in the OS.

wgui/src/parser/style.rs

@@ -65,6 +65,7 @@ pub fn parse_text_style(attribs: &[AttribPair]) -> TextStyle {
 				}
 			},
 			"weight" => match value {
+				"light" => style.weight = Some(FontWeight::Light),
 				"normal" => style.weight = Some(FontWeight::Normal),
 				"bold" => style.weight = Some(FontWeight::Bold),
 				_ => {