Implement a WebView

[stackotter]

Web views are used by many applications and are also tricky to do in a cross-platform way. This makes them a perfect candidate for being a first party offering of SwiftCrossUI! Due to being a bit of a "you either need it or you don't" feature and requires additional dependencies on Linux, we probably want to pop this behind a trait once we bump to a Swift version that supports them.

This issue requires:

• Designing a declarative API for displaying and controlling a web view, likely using bindings for things such as the current URL, and modifiers for controlling settings such as whether JS is enabled etc.
• Updating the AppBackend protocol with suitable webview-related methods that neatly abstract over all of the common functionality that we want to wrap
• Updating the underlying backend implementations to implement these new protocol requirements
• Implementing a WebView struct conforming to View and using the underlying backend to implement the designed API

Underlying technologies:

• On Apple platforms we can wrap WKWebView
• On Linux we can use WebKitGTK
• On Windows we can use WebView2

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If you're tackling this issue, I'd recommend settling on an API design before getting started on the implementation and commenting on this issue with some details so that I can give some feedback or guidance. Alternatively, join the SwiftCrossUI Discord server and we can discuss things there.