Write all the content of the array buffer (arr) to the writer (w).

// Example writing to stdout
const contentBytes = new TextEncoder().encode("Hello World");
await writeAll(Deno.stdout, contentBytes);

// Example writing to file
const contentBytes = new TextEncoder().encode("Hello World");
const file = await Deno.open('test.file', {write: true});
await writeAll(file, contentBytes);
Deno.close(file.rid);

// Example writing to buffer
const contentBytes = new TextEncoder().encode("Hello World");
const writer = new Buffer();
await writeAll(writer, contentBytes);
console.log(writer.bytes().length);  // 11