Added Studio hotload command

- /iris studio hotload and its refresh alias reload the open Studio pack on Bukkit
- Bukkit hotload refuses while Studio entry, pause or close is pending
- Modded servers register the same hotload and refresh commands
- Modded hotload reports busy while that Studio is already hotloading
- Hotload messages and help entries are localized in every language file
- Bukkit artifact verification reads the jar task output
- Multiplexor takes the Bukkit artifact from the packedJar task
- BukkitTestServer shares one-time registry setup across tests
- Tests and a gameplay scenario cover both hotload commands

gradle/multiplexor.gradle

@@ -34,8 +34,7 @@ project.ext.multiplexor = [
 ]
 
 gradle.projectsEvaluated {
-    File bukkitSource = tasks.named('jar', Jar).get().archiveFile.get().asFile
-    File bukkitArtifact = new File(bukkitSource.parentFile, bukkitSource.name.replaceFirst(/\.jar$/, '-packed.jar'))
+    File bukkitArtifact = tasks.named('packedJar').get().outputJar.get().asFile
     File apiArtifact = tasks.named('irisApi', GenerateApiTask).get().outputFile.locationOnly.get().asFile
     File spiArtifact = project(':spi').tasks.named('jar', Jar).get().archiveFile.locationOnly.get().asFile
     buildTargets.addAll([