[FIR JS] Implement FirJsExternalChecker

The JsAllowValueClassesInExternals feature is enabled explicitly,
because otherwise it's enabled
implicitly depending on the backend. See:
org/jetbrains/kotlin/test/builders/LanguageVersionSettingsBuilder.kt:90

A property may have a fake source return kind, while its accessor
has a real source kind. In this case we can't "just copy"
the property return type down to the accessor.

compiler/testData/diagnostics/testsWithJsStdLib/native/innerClass.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 external class C {
     inner class <!WRONG_EXTERNAL_DECLARATION!>Inner<!>
 }
@@ -6,4 +7,4 @@ external enum class E {
     X;
 
     inner class <!WRONG_EXTERNAL_DECLARATION!>Inner<!>
-}
+}