Refactor source layout

Introduce compat source sets and move all kotlin sources to the main directory

[no changelog]

src/main/kotlin/events/ClientStartedEvent.kt

@@ -0,0 +1,6 @@
+
+package moe.nea.firmament.events
+
+class ClientStartedEvent : FirmamentEvent() {
+    companion object : FirmamentEventBus<ClientStartedEvent>()
+}