builtin: add methods to builtin channels (#6303)

vlib/builtin/chan.v

@@ -0,0 +1,20 @@
+module builtin
+
+enum ChanState {
+	success
+	not_ready // push()/pop() would have to wait, but no_block was requested
+	closed
+}
+
+// The following methods are only stubs. The real implementation
+// is in `vlib/sync/channels.v`
+
+pub fn (ch chan) close() {}
+
+pub fn (ch chan) try_pop(obj voidptr) ChanState {
+	return .success
+}
+
+pub fn (ch chan) try_push(obj voidptr) ChanState {
+	return .success
+}