xilem_web: Add open and save file example #2398

Job	Run time
cargo clippy (windows-latest)	5m 55s
cargo check (msrv) (windows-latest)	2m 17s
cargo check (aarch64-android)	31s
cargo clippy (macos-latest)	3m 17s
cargo check (msrv) (wasm32)	27s
cargo check (msrv) (macos-latest)	1m 0s
cargo clippy (ubuntu-latest)	4m 4s
cargo check (msrv) (ubuntu-latest)	59s
cargo clippy (wasm32)	2m 23s
cargo doc	1m 0s
formatting	28s
Prime LFS Cache	4s
cargo test (wasm32)	31s
typos	6s
cargo test (windows-latest)	3m 47s
cargo test (macos-latest)	2m 12s
cargo test (ubuntu-latest)	4m 22s
	33m 23s

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