[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/nested.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 object O
 
 class TopLevel {
@@ -39,4 +40,3 @@ fun topLevelFun() {
 
     <!NESTED_EXTERNAL_DECLARATION!>external fun bar(): Int<!>
 }
-