Switch to Forge's DeferredRegister

Well, mostly. We currently don't do recipe serializers as I'm a little
too lazy. For items, blocks and TE types this does make registration
nicer - we've some helper functions which help reduce duplication.

Some types (containers, TEs, etc..) are a little less nice, as we now
must define the registry object (i.e. the WhateverType<?>) in a separate
class to the class it constructs. However, it's probably a worthwhile
price to pay.

src/main/java/dan200/computercraft/shared/Config.java

@@ -270,7 +270,7 @@ public final class Config
         clientSpec = clientBuilder.build();
     }
 
-    public static void load()
+    public static void setup()
     {
         ModLoadingContext.get().registerConfig( ModConfig.Type.SERVER, serverSpec );
         ModLoadingContext.get().registerConfig( ModConfig.Type.CLIENT, clientSpec );