vim-patch:8.2.2658: :for cannot loop over a string

Problem:    :for cannot loop over a string.
Solution:   Accept a string argument and iterate over its characters.
74e54fcb44

v8.2.2659 is already ported.

N/A patches for version.c:

vim-patch:8.2.2736: Vim9: for loop over string is a bit slow

Problem:    Vim9: for loop over string is a bit slow.
Solution:   Avoid using strlen().
175a41c13f

src/nvim/testdir/test_vimscript.vim

@@ -1692,6 +1692,26 @@ func Test_function_defined_line()
     call delete('Xtest.vim')
 endfunc
 
+func Test_for_over_string()
+  let res = ''
+  for c in 'aéc̀d'
+    let res ..= c .. '-'
+  endfor
+  call assert_equal('a-é-c̀-d-', res)
+
+  let res = ''
+  for c in ''
+    let res ..= c .. '-'
+  endfor
+  call assert_equal('', res)
+
+  let res = ''
+  for c in v:_null_string
+    let res ..= c .. '-'
+  endfor
+  call assert_equal('', res)
+endfunc
+
 "-------------------------------------------------------------------------------
 " Modelines								    {{{1
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker