Reorganize stdlib-js sources specific to the current JS backend

Move kotlin-stdlib-js project and the sources specific to the current backend to 'stdlib/js-v1' directory,
but leave sources that can be shared with the new IR backend in the common 'stdlib/js' location
with exception for 'stdlib/js/src/generated', which is used exclusively for current backend.
This simplifies sourceset configuration when building stdlib with the new backend.

libraries/stdlib/js-ir/runtime/irTestsOptionalExpectation.kt

@@ -0,0 +1,7 @@
+package kotlin
+
+
+annotation class OptionalExpectationDummy()
+
+// in order to make the compiler ignore semantics of the real OptionalExpectation
+typealias OptionalExpectation = OptionalExpectationDummy