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.

js/js.tests/test/org/jetbrains/kotlin/js/test/JsTestChecker.kt

@@ -154,7 +154,7 @@ object NashornJsTestChecker : AbstractNashornJsTestChecker() {
 class NashornIrJsTestChecker : AbstractNashornJsTestChecker() {
     override val preloadedScripts = listOf(
         BasicBoxTest.TEST_DATA_DIR_PATH + "nashorn-polyfills.js",
-        "libraries/stdlib/js/src/js/polyfills.js"
+        "libraries/stdlib/js-v1/src/js/polyfills.js"
     )
 }