[JS] JsExport diagnostics and legacy support

Account for JsExport in legacy backend namer. It means we
catch overloaded exported function conflicts for free!

Add error diagnostics:
* NESTED_JS_EXPORT (Fixes KT-36798)
* WRONG_EXPORTED_DECLARATION (Part of the fix for KT-37752)
* NON_EXPORTABLE_TYPE (Fixes KT-37771)

compiler/testData/diagnostics/testsWithJsStdLib/export/jsExportOnNestedDeclarations.kt

@@ -0,0 +1,22 @@
+// !USE_EXPERIMENTAL: kotlin.js.ExperimentalJsExport
+
+package foo
+
+class C1 {
+    <!NESTED_JS_EXPORT!>@JsExport<!>
+    fun f1() {}
+
+    <!NESTED_JS_EXPORT!>@JsExport<!>
+    val p: Int = 10
+
+    <!NESTED_JS_EXPORT!>@JsExport<!>
+    object O
+}
+
+fun f2() {
+    <!NESTED_JS_EXPORT!>@JsExport<!>
+    fun f3() {}
+
+    <!NESTED_JS_EXPORT!>@JsExport<!>
+    class C2
+}