Store web_sys::File inside of DroppedFile (#8354)

* 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.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

Cargo.lock

@@ -1284,7 +1284,6 @@ dependencies = [
  "serde",
  "static_assertions",
  "wasm-bindgen",
- "wasm-bindgen-futures",
  "web-sys",
  "web-time",
  "wgpu",
@@ -1311,6 +1310,7 @@ dependencies = [
  "serde",
  "smallvec",
  "unicode-segmentation",
+ "web-sys",
 ]
 
 [[package]]