fix: yank whole lines with yy and preserve paste shape

tests/line_yank.rs

@@ -0,0 +1,16 @@
+use vamp::editor::{Editor, Effect, Input};
+#[test]
+fn yy_copies_current_whole_line_for_paste_below_it() {
+    let mut editor = Editor::open(None, vec![]).unwrap();
+    editor.set_clipboard_mode("separate").unwrap();
+    editor
+        .apply(vec![Effect::Replace("first\nsecond\n".into())])
+        .unwrap();
+    for key in "llyy".chars() {
+        editor.handle(Input::Char(key));
+    }
+    assert_eq!(editor.cursor().column, 2);
+    assert_eq!(editor.context().text, "first\nsecond\n");
+    editor.handle(Input::Char('p'));
+    assert_eq!(editor.context().text, "first\nfirst\nsecond\n");
+}