vim-patch:9.1.1284: not possible to configure pum truncation char

Problem:  not possible to configure the completion menu truncation
          character
Solution: add the "trunc" suboption to the 'fillchars' setting to
          configure the truncation indicator (glepnir).

closes: vim/vim#17006

b87620466c

Co-authored-by: glepnir <glephunter@gmail.com>

src/nvim/buffer_defs.h

@@ -1056,6 +1056,7 @@ typedef struct {
   schar_T msgsep;
   schar_T eob;
   schar_T lastline;
+  schar_T trunc;
 } fcs_chars_T;
 
 /// Structure which contains all information that belongs to a window.