patch 8.2.3820: "vrc" does not replace composing characters

Problem:    "vrc" does not replace composing characters, while "rc" does.
Solution:   Check the byte length including composing characters.
            (closes #9351)

src/testdir/test_visual.vim

@@ -234,6 +234,10 @@ func Test_virtual_replace()
   call assert_equal("\txaaaa", getline(1))
   set softtabstop&
 
+  call setline(1, "xã̳x")
+  normal gg0lvrb
+  call assert_equal("xbx", getline(1))
+
   enew!
   set noai bs&vim
   if exists('save_t_kD')