Deprecate old default_theme function

src/assets.rs

@@ -90,6 +90,7 @@ impl HighlightingAssets {
     ///
     /// See <https://github.com/sharkdp/bat/issues/1746> and
     /// <https://github.com/sharkdp/bat/issues/1928> for more context.
+    #[deprecated(note = "use bat::theme::theme instead")]
     pub fn default_theme() -> &'static str {
         #[cfg(not(target_os = "macos"))]
         {