Add syntax highlighing feature to egui_extras (#3333)

* Add syntax highlighing feature to egui_extras

Enable "syntect" feature for great syntax highlighting of any language.

If not a simple fallback is used that works fine for C++, Rust, Python

* Check --no-default-features of egui_extras on CI

* spelling

* Fix building egui_extras without additional features

crates/egui_extras/src/lib.rs

@@ -13,6 +13,8 @@
 #[cfg(feature = "chrono")]
 mod datepicker;
 
+pub mod syntax_highlighting;
+
 #[doc(hidden)]
 pub mod image;
 mod layout;