cmdline: CmdlineEnter and CmdlineLeave autocommands (#7422)

vim-patch:fafcf0dd59fd

patch 8.0.1206: no autocmd for entering or leaving the command line

Problem:    No autocmd for entering or leaving the command line.
Solution:   Add CmdlineEnter and CmdlineLeave.

fafcf0dd59

src/nvim/version.c

@@ -78,6 +78,7 @@ static char *features[] = {
 
 // clang-format off
 static const int included_patches[] = {
+  1206,
   // 1026,
   1025,
   1024,