[Wasm] Add @JsExport diagnostics test

@JsExport is only allowed on top-level functions in K/Wasm

Restrictions for nested function are handled by reusing K/JS diagnostics
Restrictions for classes and properties are handled by specifying
 annotation target in stdlib

compiler/testData/diagnostics/wasmTests/jsInterop/jsExport.kt

@@ -0,0 +1,25 @@
+// !OPT_IN: kotlin.js.ExperimentalJsExport
+
+@JsExport
+fun foo1() {
+}
+
+class C {
+    <!NESTED_JS_EXPORT!>@JsExport<!>
+    fun memberFunction() {
+    }
+}
+
+fun foo2() {
+    <!NESTED_JS_EXPORT!>@JsExport<!>
+    fun localFun() {
+    }
+}
+
+val p1 = (<!NESTED_JS_EXPORT!>@JsExport<!> fun () {})
+
+<!WRONG_ANNOTATION_TARGET!>@JsExport<!>
+class C2
+
+<!WRONG_ANNOTATION_TARGET!>@JsExport<!>
+var p2: Int = 1