[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/wrongExportedDeclaration.kt

@@ -0,0 +1,20 @@
+// !USE_EXPERIMENTAL: kotlin.js.ExperimentalJsExport
+// !RENDER_DIAGNOSTICS_MESSAGES
+
+package foo
+
+<!WRONG_EXPORTED_DECLARATION("inline function with reified type parameters")!>@JsExport
+inline fun <reified T> inlineReifiedFun(x: Any)<!> = x is T
+
+<!WRONG_EXPORTED_DECLARATION("suspend function")!>@JsExport
+suspend fun suspendFun()<!> { }
+
+<!WRONG_EXPORTED_DECLARATION("extension property")!>@JsExport
+val String.extensionProperty<!>
+    get() = this.length
+
+@JsExport
+enum class <!WRONG_EXPORTED_DECLARATION("enum class")!>EnumClass<!> { ENTRY1, ENTRY2 }
+
+@JsExport
+annotation class <!WRONG_EXPORTED_DECLARATION("annotation class")!>AnnotationClass<!>