[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/enumEntry.fir.kt

@@ -1,7 +1,7 @@
 external enum class E {
     X,
-    Y {
+    <!EXTERNAL_ENUM_ENTRY_WITH_BODY!>Y {
         fun foo()
-    },
-    Z {}
+    },<!>
+    <!EXTERNAL_ENUM_ENTRY_WITH_BODY!>Z {}<!>
 }