Fix redstone relay not being registered as a peripheral

This was only present on the 1.21 NF version, hence not being noticed
before. Fixes #2020.

I pruned my Gradle cache recently, and I'm on some truly terrible hotel
wifi, so this is entirely untested. No beta, we die like men.

projects/forge/src/main/java/dan200/computercraft/ComputerCraft.java

@@ -156,6 +156,7 @@ public static void onRegisterCapabilities(RegisterCapabilitiesEvent event) {
         event.registerBlockEntity(PeripheralCapability.get(), ModRegistry.BlockEntities.WIRELESS_MODEM_ADVANCED.get(), WirelessModemBlockEntity::getPeripheral);
         event.registerBlockEntity(PeripheralCapability.get(), ModRegistry.BlockEntities.WIRED_MODEM_FULL.get(), WiredModemFullBlockEntity::getPeripheral);
         event.registerBlockEntity(PeripheralCapability.get(), ModRegistry.BlockEntities.CABLE.get(), CableBlockEntity::getPeripheral);
+        event.registerBlockEntity(PeripheralCapability.get(), ModRegistry.BlockEntities.REDSTONE_RELAY.get(), (b, d) -> b.peripheral());
 
         event.registerBlockEntity(WiredElementCapability.get(), ModRegistry.BlockEntities.WIRED_MODEM_FULL.get(), (b, d) -> b.getElement());
         event.registerBlockEntity(WiredElementCapability.get(), ModRegistry.BlockEntities.CABLE.get(), CableBlockEntity::getWiredElement);