[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.

js/js.translator/testData/box/native/passExtLambdaToNative.kt

@@ -1,4 +1,5 @@
 // IGNORE_BACKEND_K1: JS_IR
+// IGNORE_BACKEND_K2: JS_IR
 // IGNORE_BACKEND: JS_IR_ES6
 // TODO: Unmute when extension functions are supported in external declarations.
 // IGNORE_BACKEND: JS

js/js.translator/testData/box/native/passMemberOrExtToNative.kt

@@ -1,4 +1,5 @@
 // IGNORE_BACKEND_K1: JS_IR
+// IGNORE_BACKEND_K2: JS_IR
 // IGNORE_BACKEND: JS_IR_ES6
 // TODO: Unmute when extension functions are supported in external declarations.
 // IGNORE_BACKEND: JS