fix(column)!: ensure 'statuscolumn' works with virtual and wrapped lines

Problem:    The `'statuscolumn'` was not re-evaluated for wrapped lines,
            when preceded by virtual/filler lines. There was also no way
            to distinguish virtual and wrapped lines in the status column.
Solution:   Make sure to rebuild the statuscolumn, and replace variable
            `v:wrap` with `v:virtnum`. `v:virtnum` is negative when drawing
            virtual lines, zero when drawing the actual buffer line, and
            positive when drawing the wrapped part of a buffer line.

src/nvim/eval.c

@@ -268,7 +268,7 @@ static struct vimvar {
   VV(VV__NULL_BLOB,       "_null_blob",       VAR_BLOB, VV_RO),
   VV(VV_LUA,              "lua",              VAR_PARTIAL, VV_RO),
   VV(VV_RELNUM,           "relnum",           VAR_NUMBER, VV_RO),
-  VV(VV_WRAP,             "wrap",             VAR_BOOL, VV_RO),
+  VV(VV_VIRTNUM,          "virtnum",          VAR_NUMBER, VV_RO),
 };
 #undef VV