patch 8.2.0818: Vim9: using a discovery phase doesn't work well

Problem:    Vim9: using a discovery phase doesn't work well.
Solution:   Remove the discovery phase, instead compile a function only when
            it is used.  Add :defcompile to compile def functions earlier.

src/ex_docmd.c

@@ -273,6 +273,8 @@ static void	ex_tag_cmd(exarg_T *eap, char_u *name);
 # define ex_continue		ex_ni
 # define ex_debug		ex_ni
 # define ex_debuggreedy		ex_ni
+# define ex_def			ex_ni
+# define ex_defcompile		ex_ni
 # define ex_delfunction		ex_ni
 # define ex_disassemble		ex_ni
 # define ex_echo		ex_ni