Bundle java source with jar (#553)

matches wpilibsuite/vscode-wpilib#589

src/main/resources/export/java/build.gradle

@@ -88,6 +88,7 @@ wpi.sim.addDriverstation()
 // knows where to look for our Robot Class.
 jar {
     from { configurations.runtimeClasspath.collect { it.isDirectory() ? it : zipTree(it) } }
+    from sourceSets.main.allSource
     manifest edu.wpi.first.gradlerio.GradleRIOPlugin.javaManifest(ROBOT_MAIN_CLASS)
     duplicatesStrategy = DuplicatesStrategy.INCLUDE
 }