vim-patch:9.0.2056: no digraph for quadruple prime

Problem:  no digraph for quadruple prime
Solution: add quadruple prime digraph using 4'

closes: vim/vim#13380

47416d1a74

Co-authored-by: Jonathan Wright <quaggy@gmail.com>

test/old/testdir/test_digraph.vim

@@ -37,6 +37,9 @@ func Test_digraphs()
   call Put_Dig("=P")
   call Put_Dig("P=")
   call assert_equal(['Р']+repeat(["₽"],2)+['П'], getline(line('.')-3,line('.')))
+  " Quadruple prime
+  call Put_Dig("'4")
+  call assert_equal("⁗", getline('.'))
   " Not a digraph
   call Put_Dig("a\<bs>")
   call Put_Dig("\<bs>a")