implement base64url in Buffer

### Environment

latest node and unenv

### Reproduction

Buffer.from('hello').toString('base64url')

### Describe the bug

base64url is not implemented

### Additional context

base64url is just base64.replaceAll("+", "-").replaceAll("/", "_").replace("==", "")

there's https://github.com/feross/buffer/pull/314 but the author is not proving it

### Logs

```sh

```