[Wasm] Port external declaration checker to K2 (KT-56849)

Share common code with FirJsExternalChecker using
FirWebCommonExternalChecker

compiler/testData/diagnostics/wasmTests/jsInterop/delegation.kt

@@ -0,0 +1,18 @@
+external interface I
+
+external object O : I
+
+
+class Delegate {
+    operator fun getValue(thisRef: Any?, property: Any): String = ""
+
+    operator fun setValue(thisRef: Any?, property: Any, value: String) {}
+}
+
+external class A : <!EXTERNAL_DELEGATION!>I by O<!> {
+    val prop <!EXTERNAL_DELEGATION!>by Delegate()<!>
+
+    var mutableProp <!EXTERNAL_DELEGATION!>by Delegate()<!>
+}
+
+external val topLevelProp <!EXTERNAL_DELEGATION!>by Delegate()<!>