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.

uidev/src/testbed/testbed_any.rs

@@ -5,6 +5,7 @@ use crate::{
 use glam::Vec2;
 use wgui::{
 	assets::AssetPath,
+	font_config::WguiFontConfig,
 	globals::WguiGlobals,
 	layout::{LayoutParams, RcLayout},
 	parser::{ParseDocumentParams, ParserState},
@@ -24,6 +25,7 @@ impl TestbedAny {
 		let globals = WguiGlobals::new(
 			Box::new(assets::Asset {}),
 			wgui::globals::Defaults::default(),
+			&WguiFontConfig::default(),
 		)?;
 
 		let (layout, state) = wgui::parser::new_layout_from_assets(