vim-patch:9.0.1717: virtcol2col returns last byte of a multi-byte char (#24729)

Problem: virtcol2col returns last byte of a multi-byte char
Solution: Make it return the first byte for a multi-byte char

closes: vim/vim#12786
closes: vim/vim#12799

b209b86e66

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

src/nvim/eval.lua

@@ -11931,6 +11931,9 @@ M.funcs = {
       {lnum}, then the byte index of the character at the last
       virtual column is returned.
 
+      For a multi-byte character, the column number of the first
+      byte in the character is returned.
+
       The {winid} argument can be the window number or the
       |window-ID|. If this is zero, then the current window is used.