Add bindings for context menu API

[fvito]

Hello, this PR adds support for the Chrome's ContextMenus API

According to the Firefox docs, Firefox should also support this API to the same level as Chrome, but they also have additional properties. I'm not sure how best to support them as well have a different namespace, menus vs contextMenus

There is also a case where certain properties return number|string which I was not sure how to model them as Option<JsValue> is not possible. One option would be drop the Option part and use just JsValue

I also have the structs for the OnClickData and the createProperties object, but I'm not sure if they should be in this crate or the web-extensions one.

Also, I'm not that familiar with Rust yet, so let me know if anything needs changes

[flosse]

I'm currently in a completely different world, so I can't really judge this PR, but I would say it's good enough to start with :-)