Pass HtmlCanvasElement element directly in WebRunner::start

[jprochazk]

• Closes WebRunner::start should accept canvas object #4746

Migration guide

WebRunner::start now accepts a web_sys::HtmlCanvasElement instead of its ID.

If you are responsible for creating a canvas and assigning it an ID, you no longer need to assign it an ID, and can instead directly pass the canvas to WebRunner:

let canvas = document.createElement("canvas");
body.appendChild(canvas);

let handle = new WebHandle();
handle.start(canvas);

If you already have a canvas in your HTML, you are no longer forced to assign it an ID. You can now use any method you prefer to retrieve it from the DOM.

<canvas id="the_canvas"></canvas>
<script>
let handle = new WebHandle();
handle.start(the_canvas);
</script>

[emilk]

I think we should get this over the finish line.

Let's not bother with backwards compatibility (i.e. handling either an id or an element).

[jprochazk]

CI failure seems to be coming from main