Add back customManifest api

src/main/java/net/fabricmc/loom/LoomGradleExtension.java

@@ -98,11 +98,6 @@ public interface LoomGradleExtension extends LoomGradleExtensionAPI {
 		return Boolean.parseBoolean(System.getProperty("idea.sync.active", "false"));
 	}
 
-	default String getCustomManifest() {
-		// TODO reimplement
-		return null;
-	}
-
 	default String getIntermediaryUrl(String minecraftVersion) {
 		// TODO reimplement a way to change this, was never really supported api anyway
 		return String.format("https://maven.fabricmc.net/net/fabricmc/intermediary/%1$s/intermediary-%1$s-v2.jar", minecraftVersion);

src/main/java/net/fabricmc/loom/api/LoomGradleExtensionAPI.java

@@ -80,4 +80,8 @@ public interface LoomGradleExtensionAPI {
 	void runs(Action<NamedDomainObjectContainer<RunConfigSettings>> action);
 
 	NamedDomainObjectContainer<RunConfigSettings> getRunConfigs();
+
+	void setCustomManifest(String customManifest);
+
+	String getCustomManifest();
 }

src/main/java/net/fabricmc/loom/extension/LoomGradleExtensionApiImpl.java

@@ -57,6 +57,7 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 	protected boolean shareCaches = false;
 	protected String refmapName = null;
 	protected boolean remapMod = true;
+	protected String customManifest;
 
 	private NamedDomainObjectContainer<RunConfigSettings> runConfigs;
 
@@ -158,6 +159,17 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 		return remapMod;
 	}
 
+	@Override
+	public void setCustomManifest(String customManifest) {
+		Objects.requireNonNull(customManifest, "Custom manifest cannot be null");
+		this.customManifest = customManifest;
+	}
+
+	@Override
+	public String getCustomManifest() {
+		return customManifest;
+	}
+
 	protected abstract Project getProject();
 
 	protected abstract LoomFiles getFiles();