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

embassy-macros/Cargo.toml

@@ -18,3 +18,4 @@ nrf = []
 stm32 = []
 rp = []
 std = []
+wasm = []