Refactor source layout

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

[no changelog]

src/main/kotlin/util/Base64Util.kt

@@ -0,0 +1,10 @@
+
+package moe.nea.firmament.util
+
+object Base64Util {
+    fun String.padToValidBase64(): String {
+        val align = this.length % 4
+        if (align == 0) return this
+        return this + "=".repeat(4 - align)
+    }
+}