Additional work towards 1.16 compatibility

- Very basic implementation of the SideEffects system. Will definitely need fine tuning for it to be functional, but is not considered a priority in my opinion.
- Minor changes to the World interface and World implementations related to the SideEffects system. Shouldn't be the cause of any new bugs but be on the lookout.
- Included debug in BukkitImplLoader.java to assist contributors in understanding what needs to be implemented for the adapter to load properly.

Still very WIP but we're a few steps closer. So far, this is coming along better than I anticipated. Hopefully we can keep the momentum.

worldedit-forge/build.gradle.kts

@@ -77,7 +77,12 @@ tasks.named<Copy>("processResources") {
     }
 }
 
-addJarManifest(includeClasspath = false)
+tasks.named<Jar>("jar") {
+    manifest {
+        attributes("Class-Path" to CLASSPATH,
+                "WorldEdit-Version" to project.version)
+    }
+}
 
 tasks.named<ShadowJar>("shadowJar") {
     dependencies {