vim-patch:9.0.2032: cannot get mouse click pos for tab or virt text (#25653)

Problem:  Cannot accurately get mouse clicking position when clicking on
          a TAB or with virtual text.
Solution: Add a "coladd" field to getmousepos() result.

closes: vim/vim#13335

f5a94d5165

src/nvim/eval.lua

@@ -4034,6 +4034,8 @@ M.funcs = {
       	wincol		column inside "winid"
       	line		text line inside "winid"
       	column		text column inside "winid"
+      	coladd		offset (in screen columns) from the
+      			start of the clicked char
       All numbers are 1-based.
 
       If not over a window, e.g. when in the command line, then only