[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/secondaryConstructorWithoutJsName.kt

@@ -0,0 +1,15 @@
+// !USE_EXPERIMENTAL: kotlin.js.ExperimentalJsExport
+// !RENDER_DIAGNOSTICS_MESSAGES
+
+package foo
+
+@JsExport
+class C(val x: String) {
+    <!WRONG_EXPORTED_DECLARATION("secondary constructor without @JsName")!>constructor(x: Int)<!>: this(x.toString())
+}
+
+@JsExport
+class C2(val x: String) {
+    @JsName("JsNameProvided")
+    constructor(x: Int): this(x.toString())
+}