[Wasm] Move external RTT checkers to web.common and reuse in Wasm

#KT-56849

compiler/testData/diagnostics/wasmTests/jsInterop/rtti/castToNativeInterface.kt

@@ -0,0 +1,8 @@
+// FIR_IDENTICAL
+// DIAGNOSTICS: +UNCHECKED_CAST_TO_EXTERNAL_INTERFACE
+
+external interface I
+
+fun box(a: Any, b: Any): Pair<I, I?> {
+    return Pair(<!UNCHECKED_CAST_TO_EXTERNAL_INTERFACE!>a as I<!>, <!UNCHECKED_CAST_TO_EXTERNAL_INTERFACE!>b as? I<!>)
+}