vim-patch:8.2.3969: value of MAXCOL not available in Vim script

Problem:    Value of MAXCOL not available in Vim script.
Solution:   Add v:maxcol. (Naohiro Ono, closes vim/vim#9451)

56200eed62

The variable is always 2147483647, but introducing it makes functions
easier to document.

Co-authored-by: naohiro ono <obcat@icloud.com>

src/nvim/eval.h

@@ -157,6 +157,7 @@ typedef enum {
   VV_ARGV,
   VV_COLLATE,
   VV_EXITING,
+  VV_MAXCOL,
   // Nvim
   VV_STDERR,
   VV_MSGPACK_TYPES,