patch 8.2.2454: leading space can not be made visible

Problem:    Leading space can not be made visible.
Solution:   Add "lead:" to 'listchars'. (closes #7772)
diff --git a/src/screen.c b/src/screen.c
index 60544b6..cd727ed 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -4775,6 +4775,7 @@
 	{&lcs_space,	"space"},
 	{&lcs_tab2,	"tab"},
 	{&lcs_trail,	"trail"},
+	{&lcs_lead,	"lead"},
 #ifdef FEAT_CONCEAL
 	{&lcs_conceal,	"conceal"},
 #else