feat: add syntax-aware diff backgrounds and diagonal gap shading

src/syntax.rs

@@ -156,6 +156,11 @@ impl SyntaxCache {
         self.error.as_deref()
     }
 
+    #[cfg(test)]
+    pub(crate) fn parse_counts(&self) -> (usize, usize) {
+        (self.parses, self.injection_parses)
+    }
+
     fn highlight(
         &mut self,
         syntax: &'static LanguageSpec,