feat(ui): add 'statuscolumn' option

Problem:    Unable to customize the column next to a window ('gutter').
Solution:   Add 'statuscolumn' option that follows the 'statusline' syntax,
	    allowing to customize the status column. Also supporting the %@
	    click execute function label. Adds new items @C and @s which
	    will print the fold and sign columns. Line numbers and signs
	    can be clicked, highlighted, aligned, transformed, margined etc.

src/nvim/api/vim.c

@@ -2238,6 +2238,7 @@ Dictionary nvim_eval_statusline(String str, Dict(eval_statusline) *opts, Error *
                                fillchar,
                                maxwidth,
                                hltab_ptr,
+                               NULL,
                                NULL);
 
   PUT(result, "width", INTEGER_OBJ(width));