[JS IR BE] Implement private members lowering to extract private methods from class and transform them into static function

* fix kotlin.Long
 * update tests

compiler/testData/codegen/box/traits/privateInterfaceMethod.kt

@@ -0,0 +1,14 @@
+// IGNORE_BACKEND: JS
+// IGNORE_BACKEND: JVM_IR
+
+interface A {
+    private fun foo() = "OK"
+
+    public fun bar() = foo()
+}
+
+class B : A {
+    private fun foo() = "fail"
+}
+
+fun box() = B().bar()