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/lib.rs

@@ -27,6 +27,7 @@ mod assets_internal;
 pub mod components;
 pub mod drawing;
 pub mod event;
+pub mod font_config;
 pub mod gfx;
 pub mod globals;
 pub mod i18n;