style: add component 'rule' for horizontal file delimiter

tests/snapshots/output/changes_grid_header_rule.snapshot.txt

@@ -0,0 +1,26 @@
+──┬─────────────────────────────────────────────────────────────────────────────
+  │ File: sample.rs
+──┼─────────────────────────────────────────────────────────────────────────────
+  │ struct Rectangle {
+  │     width: u32,
+  │     height: u32,
+  │ }
+  │ 
+_ │ fn main() {
+  │     let rect1 = Rectangle { width: 30, height: 50 };
+  │ 
+  │     println!(
+~ │         "The perimeter of the rectangle is {} pixels.",
+~ │         perimeter(&rect1)
+  │     );
++ │     println!(r#"This line contains invalid utf8:  "�����"#;
+  │ }
+  │ 
+  │ fn area(rectangle: &Rectangle) -> u32 {
+  │     rectangle.width * rectangle.height
+  │ }
++ │ 
++ │ fn perimeter(rectangle: &Rectangle) -> u32 {
++ │     (rectangle.width + rectangle.height) * 2
++ │ }
+──┴─────────────────────────────────────────────────────────────────────────────