Buffer#fill should accept string arguments

[SheetJSDev]

Repro:

const A = Buffer.allocUnsafe(26).fill('!');
const B = Buffer.allocUnsafe(26).fill("!".charCodeAt(0));
console.log(A); console.log(B);

Node shows two Buffers filled with 0x21 bytes, while Bun does not fill in the first case.