xilem_web: Add open and save file example #2397

Job	Run time
cargo check (aarch64-android)	37s
cargo clippy (windows-latest)	7m 38s
cargo check (msrv) (wasm32)	27s
cargo clippy (macos-latest)	4m 27s
cargo check (msrv) (windows-latest)	2m 42s
cargo clippy (ubuntu-latest)	4m 46s
cargo clippy (wasm32)	4m 3s
cargo check (msrv) (macos-latest)	1m 8s
cargo doc	1m 9s
cargo check (msrv) (ubuntu-latest)	59s
formatting	30s
Prime LFS Cache	4s
cargo test (wasm32)	53s
typos	9s
cargo test (windows-latest)	4m 59s
cargo test (macos-latest)	2m 27s
cargo test (ubuntu-latest)	4m 3s
	41m 1s

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