Add WASM support for executor

* Adds an executor for WASM runtimes based on wasm_bindgen.
* Add time driver based on JS time handling.
* Add example that can run in browser locally.
* Update to critical-section version that supports 'std' flag

examples/wasm/src/lib.rs

@@ -0,0 +1,37 @@
+#![feature(type_alias_impl_trait)]
+#![allow(incomplete_features)]
+
+use embassy::{
+    executor::Spawner,
+    time::{Duration, Timer},
+};
+
+#[embassy::task]
+async fn ticker() {
+    let window = web_sys::window().expect("no global `window` exists");
+
+    let mut counter = 0;
+    loop {
+        let document = window.document().expect("should have a document on window");
+        let list = document
+            .get_element_by_id("log")
+            .expect("should have a log element");
+
+        let li = document
+            .create_element("li")
+            .expect("error creating list item element");
+        li.set_text_content(Some(&format!("tick {}", counter)));
+
+        list.append_child(&li).expect("error appending list item");
+        log::info!("tick {}", counter);
+        counter += 1;
+
+        Timer::after(Duration::from_secs(1)).await;
+    }
+}
+
+#[embassy::main]
+async fn main(spawner: Spawner) {
+    wasm_logger::init(wasm_logger::Config::default());
+    spawner.spawn(ticker()).unwrap();
+}