* Closes#4654
* Related #4667
* [x] I have followed the instructions in the PR template
This PR avoids materializing the contents of a file that was dragged
into an egui application on the web. It does so by storing the
`web_sys::File` handle directly on WASM.
This breaks the existing API of `DroppedFile` on the web, because there
is no way to retrieve the bytes synchronously form a `DroppedFile`
anymore, forcing handling call sites to become asynchronous.
The native API remains the same.
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Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>
