wasm: add a webassembly compiler backend, based on using binaryen (#17368)

examples/wasm/mandelbrot/README.md

@@ -0,0 +1,12 @@
+# V Mandelbrot Example
+
+1. First, create `mandelbrot.wasm`. Compile with `-os browser`.
+
+```
+v -b wasm -os browser mandelbrot.v
+```
+
+2. Then, open the `mandelbrot.html` file in the browser.
+	- CORS errors do not allow `mandelbrot.wasm` to be loaded.
+	- Use `python -m http.server 8080`
+	- Use `emrun mandelbrot.html`