Add MixinExtras on 1.16.4+

We'll use it in 1.20 mixins.
Unclear what minimal fabric-loader version it requires, so in the interest of
keeping older versions compatible with OptiFabric (old versions of which aren't
compatible with the latest fabric-loader), we'll add it to 1.16.4+ only.

src/main/java/com/replaymod/core/MixinExtrasInit.java

@@ -0,0 +1,14 @@
+//#if FABRIC>=1
+package com.replaymod.core;
+
+import net.fabricmc.loader.api.entrypoint.PreLaunchEntrypoint;
+
+public class MixinExtrasInit implements PreLaunchEntrypoint {
+    @Override
+    public void onPreLaunch() {
+        //#if MC>=11604
+        com.llamalad7.mixinextras.MixinExtrasBootstrap.init();
+        //#endif
+    }
+}
+//#endif