Fix `delete_char_backward` for paired characters (#4558)

When backward-deleting a character, if this character and the following
character form a Pair, we want to delete both. However, there is a bug
that deletes both characters also if both characters are closers of some
Pair.

This commit fixes that by adding an additional check that the deleted
character should be an opener in a Pair.

Closes https://github.com/helix-editor/helix/issues/4544.

helix-term/src/commands.rs

@@ -3198,6 +3198,7 @@ pub mod insert {
                         (Some(_x), Some(_y), Some(ap))
                             if range.is_single_grapheme(text)
                                 && ap.get(_x).is_some()
+                                && ap.get(_x).unwrap().open == _x
                                 && ap.get(_x).unwrap().close == _y =>
                         // delete both autopaired characters
                         {