add web-app stub

src/lib/wasm.rs

@@ -0,0 +1,10 @@
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+extern "C" {
+    fn alert(s: &str);
+}
+#[wasm_bindgen]
+pub fn greet() {
+    alert("Hello, wasm-on-web!");
+}