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>

crates/egui-winit/src/dropped_file.rs

@@ -0,0 +1,22 @@
+use std::path::{Path, PathBuf};
+
+#[derive(Debug)]
+pub(crate) struct NativeFile {
+    path: PathBuf,
+}
+
+impl From<PathBuf> for NativeFile {
+    fn from(path: PathBuf) -> Self {
+        Self { path }
+    }
+}
+
+impl egui::DroppedFile for NativeFile {
+    fn path(&self) -> &Path {
+        &self.path
+    }
+
+    fn bytes(&self) -> Result<Vec<u8>, String> {
+        std::fs::read(&self.path).map_err(|err| err.to_string())
+    }
+}