[Wasm] Add uninstantiated MJS wrapper

It allows

* Custom imports
* Ability to skip initializer

compiler/testData/codegen/boxWasmJsInterop/imperativeWrapperUninitialised.kt

@@ -0,0 +1,44 @@
+// TARGET_BACKEND: WASM
+
+// FILE: wasmImport.kt
+import kotlin.wasm.WasmImport
+
+@WasmImport("foo")
+external fun inc(x: Int): Int
+
+@JsExport
+fun myBox(): String {
+    if (inc(5) != 6) return "KFail1"
+    return "OK"
+}
+
+var initialized: Int = 0
+
+@JsExport
+fun getInitialized(): Int = initialized
+
+fun main() {
+    initialized = 100
+}
+
+// FILE: entry.mjs
+import { instantiate } from "./index.uninstantiated.mjs";
+
+let inc = x => x + 1;
+
+let imports = {
+    "foo": { inc },
+}
+
+let { exports } = await instantiate(imports, /*runInitializer=*/false);
+if (exports.getInitialized() !== 0) {
+    throw "Fail1"
+}
+exports.__init();
+if (exports.getInitialized() !== 100) {
+    throw "Fail2"
+}
+
+if (exports.myBox() != "OK") {
+    throw "Fail3"
+}