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.

build.gradle

@@ -330,6 +330,11 @@ dependencies {
         annotationProcessor 'org.ow2.asm:asm-tree:6.2'
         annotationProcessor 'org.apache.logging.log4j:log4j-core:2.0-beta9'
     }
+
+    if (mcVersion >= 11604) {
+        shadow(annotationProcessor('com.github.LlamaLad7:MixinExtras:0.1.1'))
+    }
+
     shadow 'com.googlecode.mp4parser:isoparser:1.1.7'
     shadow 'org.apache.commons:commons-exec:1.3'
     shadow 'com.google.apis:google-api-services-youtube:v3-rev178-1.22.0', shadeExclusions