Add support for multiple Toast instances with custom IDs

[GodGotzi]

This PR introduces the ability to create multiple Toasts instances with custom IDs in the egui-toasts library. A new method with_id(id: Id) has been added to the Toasts struct, allowing developers to initialize toast groups with specific identifiers.
Key changes:

• Added with_id(id: Id) -> Self method to the Toasts implementation.
• This allows users to manage multiple independent toast groups within the same UI.

This change is useful for situations where toasts need to be categorized or grouped separately.

[urholaukkarinen]

Thanks!