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

Share common code with FirJsExternalChecker using
FirWebCommonExternalChecker

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

@@ -0,0 +1,42 @@
+// FIR_IDENTICAL
+object O
+
+class TopLevel {
+    external class <!NESTED_EXTERNAL_DECLARATION!>A<!>
+
+    class B
+
+    fun foo() = 23
+
+    <!NESTED_EXTERNAL_DECLARATION!>external fun bar(): Int<!>
+
+    val x = "a"
+
+    <!NESTED_EXTERNAL_DECLARATION!>external val y: String<!>
+
+    val O.u: String get() = "O.u"
+}
+
+external class TopLevelNative {
+    external class <!NESTED_EXTERNAL_DECLARATION!>A<!>
+
+    class B
+
+    fun foo(): Int = definedExternally
+
+    <!NESTED_EXTERNAL_DECLARATION!>external fun bar(): Int<!>
+
+    val x: String = definedExternally
+
+    <!NESTED_EXTERNAL_DECLARATION!>external val y: String<!>
+}
+
+fun topLevelFun() {
+    external class <!NESTED_EXTERNAL_DECLARATION!>A<!>
+
+    class B
+
+    fun foo() = 23
+
+    <!NESTED_EXTERNAL_DECLARATION!>external fun bar(): Int<!>
+}