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

Share common code with FirJsExternalChecker using
FirWebCommonExternalChecker

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

@@ -0,0 +1,12 @@
+// FIR_IDENTICAL
+external interface I {
+    interface J
+
+    class <!NESTED_CLASS_IN_EXTERNAL_INTERFACE!>C<!>
+
+    <!NESTED_CLASS_IN_EXTERNAL_INTERFACE!>object O<!>
+
+    enum class <!NESTED_CLASS_IN_EXTERNAL_INTERFACE, WRONG_EXTERNAL_DECLARATION!>E<!>
+
+    companion <!NESTED_CLASS_IN_EXTERNAL_INTERFACE!>object<!>
+}