Web touch support

[ActuallyHappening]

Thanks so much for this crate! It has been very useful in creating my 100% Rust projects.

One limitation which is pretty glaring is the lack of web touch support. I am happy to take a stab fixing this bug, but aren't quite sure where to start. The example supports touch (tested on iOS Safari iPad/iPhone) but is using bevy 0.8 and bevy_egui 0.15, much to old for me. https://github.com/mvlabat/bevy_egui_web_showcase

You can view my project's web release here: https://caleb-msrc-q11.netlify.app (source: https://github.com/ActuallyHappening/ChessBois/tree/master)

The web version works with mouse on my MacBook, but pressing on my iOS devices doesn't give any response from the ui. If I connect my mouse to my iPad (advanced I know!) it works fine, but obviously I want normal touch interactions to work.

Any pointers / fixes (internal or other crates) are appreciated!