Synchronously flushes any pending data operations of the given file stream to disk.

const file = Deno.openSync(
 "my_file.txt",
 { read: true, write: true, create: true },
);
Deno.writeSync(file.rid, new TextEncoder().encode("Hello World"));
Deno.fdatasyncSync(file.rid);
console.log(new TextDecoder().decode(Deno.readFileSync("my_file.txt"))); // Hello World