Refactor source layout

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

[no changelog]

src/main/kotlin/features/notifications/Notifications.kt

@@ -0,0 +1,7 @@
+
+package moe.nea.firmament.features.notifications
+
+import moe.nea.firmament.features.FirmamentFeature
+
+object Notifications  {
+}