patch 8.2.2805: Vim9: cannot use legacy syntax in Vim9 script

Problem:    Vim9: cannot use legacy syntax in Vim9 script.
Solution:   Add the :legacy command.
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index aa41ab3..819cf73 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -830,6 +830,9 @@
 EXCMD(CMD_lexpr,	"lexpr",	ex_cexpr,
 	EX_NEEDARG|EX_WORD1|EX_NOTRLCOM|EX_BANG,
 	ADDR_NONE),
+EXCMD(CMD_legacy,	"legacy",	ex_wrongmodifier,
+	EX_NEEDARG|EX_EXTRA|EX_NOTRLCOM|EX_CMDWIN|EX_LOCK_OK,
+	ADDR_NONE),
 EXCMD(CMD_lfile,	"lfile",	ex_cfile,
 	EX_TRLBAR|EX_FILE1|EX_BANG,
 	ADDR_NONE),