patch 9.1.0422: function echo_string_core() is too long

Problem:  function echo_string_core() is too long
Solution: Refactor into several smaller functions
          (Yegappan Lakshmanan)

closes: #14804

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_vim9_script.vim

@@ -2159,6 +2159,18 @@ def Test_echo_cmd()
   assert_match('^two$', g:Screenline(&lines))
 
   v9.CheckDefFailure(['echo "xxx"# comment'], 'E488:')
+
+  # Test for echoing a script local function name
+  var lines =<< trim END
+    vim9script
+    def ScriptLocalEcho()
+    enddef
+    echo ScriptLocalEcho
+  END
+  new
+  setline(1, lines)
+  assert_match('<SNR>\d\+_ScriptLocalEcho', execute('source')->split("\n")[0])
+  bw!
 enddef
 
 def Test_echomsg_cmd()