Synchronously flushes any pending data and metadata 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.ftruncateSync(file.rid, 1);
Deno.fsyncSync(file.rid);
console.log(new TextDecoder().decode(Deno.readFileSync("my_file.txt"))); // H