patch 8.2.1430: Vim9: error for missing comma instead of extra white space

Problem:    Vim9: error for missing comma instead of extra white space.
Solution:   Check if comma can be found after white space. (closes #6668)
            Also check for extra white space in literal dict. (closes #6670)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 502c66f..6c43ad2 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -2394,6 +2394,11 @@
 	    semsg(_(e_list_end), *arg);
 	    return FAIL;
 	}
+	if (*p == ',')
+	{
+	    semsg(_(e_no_white_before), ",");
+	    return FAIL;
+	}
 	if (*p == ']')
 	{
 	    ++p;