Begin work on mp3 support

Add mp3spi as a dependency.

build.gradle

@@ -18,6 +18,10 @@ processResources {
 
 dependencies {
 
+    implementation("com.googlecode.soundlibs:mp3spi:${mp3spi_version}") {
+        exclude group: 'junit', module: 'junit'
+    }
+
     implementation 'com.google.code.findbugs:jsr305:3.0.2'
 
     // To use this dependency, you need to compile bukkit by yourself