Unify Iris integrations

Iris now has built-in support for NeoForge, so we can use the same
integration on both.

We also re-enable Forge's client tests, and test Iris there too.

Fixes #1967

projects/common/src/test/java/dan200/computercraft/TestPlatformHelper.java

@@ -49,6 +49,11 @@ import java.util.function.Predicate;
 
 @AutoService({ PlatformHelper.class, ComputerCraftAPIService.class })
 public class TestPlatformHelper extends AbstractComputerCraftAPI implements PlatformHelper {
+    @Override
+    public boolean isModLoaded(String id) {
+        return false;
+    }
+
     @Override
     public ConfigFile.Builder createConfigBuilder() {
         throw new UnsupportedOperationException("Cannot create config file inside tests");