patch 8.2.0321: Vim9: ":execute" does not work yet

Problem:    Vim9: ":execute" does not work yet.
Solution:   Add ISN_EXECUTE. (closes #5699) Also make :echo work with more
            than one argument.
diff --git a/src/vim9.h b/src/vim9.h
index a977965..77bf626 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -13,7 +13,8 @@
 
 typedef enum {
     ISN_EXEC,	    // execute Ex command line isn_arg.string
-    ISN_ECHO,	    // echo isn_arg.number items on top of stack
+    ISN_ECHO,	    // echo isn_arg.echo.echo_count items on top of stack
+    ISN_EXECUTE,    // execute Ex commands isn_arg.number items on top of stack
 
     // get and set variables
     ISN_LOAD,	    // push local variable isn_arg.number