fix(column): estimate 'statuscolumn' width appropriately

Problem:    The 'statuscolumn' width is being estimated without the
            proper context. In particular, this resulted in the fact
            that a custom fold column could be included in the estimated
            `number_width()`, and doubly added when actually drawing the
            statuscolumn due to `win_col_off()` also adding the
            `'foldcolumn'` width. Resulting in a status column that is
            `'foldcolumn'` cells wider than necessary.
Solution:   Estimate 'statuscolumn' width in `get_statuscol_str()` when
            a buffer's line count has changed.

src/nvim/buffer_defs.h

@@ -1392,6 +1392,7 @@ struct window_S {
   int w_prev_fraction_row;
 
   linenr_T w_nrwidth_line_count;        // line count when ml_nrwidth_width was computed.
+  linenr_T w_statuscol_line_count;      // line count when 'statuscolumn' width was computed.
   int w_nrwidth_width;                  // nr of chars to print line count.
 
   qf_info_T *w_llist;                 // Location list for this window