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/move.c

@@ -649,7 +649,8 @@ void validate_cursor_col(void)
 // fold column and sign column (these don't move when scrolling horizontally).
 int win_col_off(win_T *wp)
 {
-  return ((wp->w_p_nu || wp->w_p_rnu) ? number_width(wp) + 1 : 0)
+  return ((wp->w_p_nu || wp->w_p_rnu || (*wp->w_p_stc != NUL)) ?
+          (number_width(wp) + (*wp->w_p_stc == NUL)) : 0)
          + (cmdwin_type == 0 || wp != curwin ? 0 : 1)
          + win_fdccol_count(wp)
          + (win_signcol_count(wp) * win_signcol_width(wp));