Add support in web for showing virtual keyboard

[will-hart]

Description

When using winit in a mobile web browser in web builds, I don't have a way to pop up the virtual keyboard, which means I can't use (most) custom GUI libraries for text input.

I came across this via bevy_simple_text_input. egui supports keyboard inputs by using a hidden input field but ideally winit would provide a way to do this.

This is possibly a duplicate of #1823 and #2260.

Relevant platforms

web with a mobile device (tablet/phone)

[daxpedda]

Closing in favor of #1823.