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.
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index 25a0853..2d86c5d 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -447,6 +447,9 @@
 EXCMD(CMD_def,		"def",		ex_function,
 	EX_EXTRA|EX_BANG|EX_SBOXOK|EX_CMDWIN,
 	ADDR_NONE),
+EXCMD(CMD_defcompile,	"defcompile",	ex_defcompile,
+	EX_SBOXOK|EX_CMDWIN|EX_TRLBAR,
+	ADDR_NONE),
 EXCMD(CMD_delcommand,	"delcommand",	ex_delcommand,
 	EX_NEEDARG|EX_WORD1|EX_TRLBAR|EX_CMDWIN,
 	ADDR_NONE),