patch 8.2.3627: difficult to know where the text starts in a window

Problem:    difficult to know where the text starts in a window. (Sergey
            Vlasov)
Solution:   Add the "textoff" entry in the result of getwininfo().
            (closes #9163)
diff --git a/src/evalwindow.c b/src/evalwindow.c
index c905715..c8c1442 100644
--- a/src/evalwindow.c
+++ b/src/evalwindow.c
@@ -407,6 +407,7 @@
 #endif
     dict_add_number(dict, "width", wp->w_width);
     dict_add_number(dict, "wincol", wp->w_wincol + 1);
+    dict_add_number(dict, "textoff", win_col_off(wp));
     dict_add_number(dict, "bufnr", wp->w_buffer->b_fnum);
 
 #ifdef FEAT_TERMINAL