New drag and drop API (#4571)

This commit implements a new API for drag and drop, with a `DataTransfer` type which abstracts over the various clipboard/drag and drop APIs across different platforms. I built this on top of #2429 although admittedly I ended up removing pretty much all of their work while I was reworking the design.

This is being built in order to help support [drag-and-drop work](https://github.com/slint-ui/slint/issues/1967) in Slint's winit backend. As part of that work, I did extensive research on how drag-and-drop and clipboard APIs are implemented across different platforms, and wrote a (still WIP) research document that can be found [here](https://gist.github.com/eira-fransham/06750cf8d25ade08d362a0ca8dfafe06).

The new API is inspired by the browser's [`DataTransfer`](https://developer.mozilla.org/en-US/docs/Web/API/DataTransfer) API.

winit-core/Cargo.toml

@@ -30,6 +30,7 @@ keyboard-types.workspace = true
 rwh_06.workspace = true
 serde = { workspace = true, optional = true }
 smol_str.workspace = true
+url.workspace = true
 
 # `wasm32-unknown-unknown` and `wasm32-none`, but not `wasm32-wasi`.
 [target.'cfg(all(target_family = "wasm", any(target_os = "unknown", target_os = "none")))'.dependencies]