feat(ui): add support to display a title in the border of a float (#20184)

add "title" and "title_pos" keys to win config dict.

src/nvim/extmark_defs.h

@@ -9,6 +9,8 @@ typedef struct {
   int hl_id;
 } VirtTextChunk;
 
+typedef kvec_t(VirtTextChunk) VirtText;
+
 typedef struct undo_object ExtmarkUndoObject;
 typedef kvec_t(ExtmarkUndoObject) extmark_undo_vec_t;