KT-2752: add diagnostic that checks whether Kotlin declarations produce conflicting names in generated JS

compiler/testData/diagnostics/testsWithJsStdLib/name/packageAndMethod.txt

@@ -0,0 +1,9 @@
+package
+
+package foo {
+    public fun bar(): kotlin.Int
+
+    package foo.bar {
+        public val x: kotlin.Int = 42
+    }
+}