feat(output): add icon_override function to FileNameColours trait

Signed-off-by: Sandro-Alessio Gierens <sandro@gierens.de>

src/output/file_name.rs

@@ -513,4 +513,6 @@ pub trait Colours: FiletypeColours {
     fn icon(&self) -> Option<Style>;
 
     fn colour_file(&self, file: &File<'_>) -> Style;
+
+    fn icon_override(&self, file: &File<'_>) -> Option<char>;
 }