web: Add Instance::create_surface_from_canvas (fix #1837)

[Herschel]

Add specialized methods on web for creating a wgpu Surface directly
from a web_sys::HTMLCanvasElement and web_sys::OffscreenCanvas.

• Instance::create_surface_from_canvas
• Instance::create_surface_from_offscreen_canvas

Connections
Fixes #1837.

Description
Currently Instance::create_surface requires a HasRawWindowHandle. The web backend uses this handle to search for a canvas with a data-raw-handle attribute.

This is handy for full-screen demos or games, or quickly compiling winit apps to both desktop and web. However, this isn't ideal for apps that are part of a larger site, or usable as a JS library. For example, the user may want to instantiate wgpu without attaching the canvas to the DOM, or the canvas may be buried inside a shadow DOM. Additionally, it'd be nice to use OffscreenCanvas. See #1837 for more info.

This adds web-specific Instance::create_surface_from_canvas and Instance::create_surface_from_offscreen_canvas methods that directly accept web_sys::HtmlCanvasElement and web_sys::OffscreenCanvas. These methods are only compiled for the wasm32 architecture. This was modeled after the Instance::create_surface_from_core_animation_layer method for macOS.

This does add web_sys types to the externally visible API. However, web_sys is already a dependency of the project, and as WebGPU is a web-based API, wgpu should work directly with standard web types.

[grovesNL]

Nice, thank you!

This seems good to me assuming we're ok with having web-sys types/version as part of the public API. I'm not sure of a way around this besides having a common canvas trait that web-sys implements (like we do with raw-window-handle)