Implement support for custom concrete items.

samples/mixed/other-library/build.gradle.kts

@@ -1,3 +1,10 @@
 plugins {
   id("gay.pizza.foundation.concrete-base")
 }
+
+concreteItem {
+  type.set("library")
+  fileInclusion {
+    tasks.jar.get().outputs.files.associateWith { "library-jar" }
+  }
+}