[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/body.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 external fun foo(): Int = definedExternally
 
 external fun bar(): Unit {
@@ -27,4 +28,4 @@ external class C {
     fun foo(): Int = definedExternally
 
     fun bar(): Int = <!WRONG_BODY_OF_EXTERNAL_DECLARATION!>23<!>
-}
+}