Force canvas/text input focus on touch for iOS web browsers

[BKSalman]

Related to #4569

• moved is_mobile from eframe/src/web/text_agent.rs to eframe/src/web/mod.rs and used it in eframe/src/web/events.rs
• added event listener for pointerdown event
• added event listener for pointerup event and forced the canvas to be focused when the event is received and therefore the on-screen keyboard appearing

This PR tries to fix iOS browsers issue where it doesn't focus the canvas/text input on touch and therefore not showing the on-screen keyboard. It does it by force focusing the canvas on pointerup

[ejjonny]

This is a big step forward for egui mobile web, ty

[emilk]

I tested it on iOS and it definitely helps #4569, but does not completely fix it. Still a step in the right direction ⭐