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/Cargo.toml

@@ -19,6 +19,7 @@ workspace = true
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--generate-link-to-definition"]
+targets = ["x86_64-unknown-linux-gnu", "wasm32-unknown-unknown"]
 
 [lib]
 
@@ -90,3 +91,9 @@ document-features = { workspace = true, optional = true }
 
 ron = { workspace = true, optional = true }
 serde = { workspace = true, optional = true, features = ["derive", "rc"] }
+
+
+# web:
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+# For `DroppedFile`, which hands web apps a file handle instead of its contents.
+web-sys = { workspace = true, features = ["File"] }