patch 8.2.0578: heredoc for interfaces does not support "trim"

Problem:    Heredoc for interfaces does not support "trim".
Solution:   Update the script heredoc support to be same as the :let command.
            (Yegappan Lakshmanan, closes #5916)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 666db15..1ccf1a7 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -4207,7 +4207,7 @@
 	// [let] varname =<< [trim] {end}
 	eap->getline = heredoc_getline;
 	eap->cookie = cctx;
-	l = heredoc_get(eap, op + 3);
+	l = heredoc_get(eap, op + 3, FALSE);
 
 	// Push each line and the create the list.
 	FOR_ALL_LIST_ITEMS(l, li)