patch 9.0.0811: error if :echowin is preceded by a command modifier

Problem:    Error if :echowin is preceded by a command modifier.
Solution:   Do not give an error for range when there is a modifier.
            (closes #11414)

src/testdir/test_vim9_script.vim

@@ -2032,6 +2032,10 @@ enddef
 def Test_echowindow_cmd()
   var local = 'local'
   echowindow 'something' local # comment
+
+  # with modifier
+  unsilent echowin 'loud'
+
   # output goes in message window
   popup_clear()
 enddef

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    811,
 /**/
     810,
 /**/

src/vim9compile.c

@@ -2712,8 +2712,9 @@ get_cmd_count(char_u *line, exarg_T *eap)
 	;
     if (!isdigit(*p))
     {
-	// the command must be following
-	if (p < eap->cmd)
+	// The command or modifiers must be following.  Assume a lower case
+	// character means there is a modifier.
+	if (p < eap->cmd && !vim_islower(*p))
 	{
 	    emsg(_(e_invalid_range));
 	    return -1;