feat(marks): add conceal_lines to nvim_buf_set_extmark()

Implement an extmark property that conceals lines vertically.

src/nvim/buffer_defs.h

@@ -813,6 +813,7 @@ typedef struct {
   uint16_t wl_size;             // height in screen lines
   char wl_valid;                // true values are valid for text in buffer
   char wl_folded;               // true when this is a range of folded lines
+  linenr_T wl_foldend;          // last buffer line number for folded line
   linenr_T wl_lastlnum;         // last buffer line number for logical line
 } wline_T;