patch 8.2.3346: Vim9: no error for using "." for concatenation after ":vim9cmd"

Problem:    Vim9: no error for using "." for concatenation after ":vim9cmd".
            (Naohiro Ono)
Solution:   Check for Vim9 script syntax. (closes #8756)

src/eval.c

@@ -2860,7 +2860,8 @@ eval5(char_u **arg, typval_T *rettv, evalarg_T *evalarg)
 	// "++" and "--" on the next line are a separate command.
 	p = eval_next_non_blank(*arg, evalarg, &getnext);
 	op = *p;
-	concat = op == '.' && (*(p + 1) == '.' || current_sctx.sc_version < 2);
+	concat = op == '.' && (*(p + 1) == '.'
+			      || (current_sctx.sc_version < 2 && !vim9script));
 	if ((op != '+' && op != '-' && !concat) || p[1] == '='
 					       || (p[1] == '.' && p[2] == '='))
 	    break;

src/testdir/test_vim9_cmd.vim

@@ -14,6 +14,7 @@ def Test_vim9cmd()
   END
   CheckScriptSuccess(lines)
   assert_fails('vim9cmd', 'E1164:')
+  assert_fails('vim9cmd echo "con" . "cat"', 'E15:')
 
   lines =<< trim END
       vim9script

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3346,
 /**/
     3345,
 /**/