[bug] Unknown encoding: base64url

```js
const NodeBuffer = require('node:buffer').Buffer;
const BrowserBuffer = require('buffer/').Buffer;

console.log(NodeBuffer.from('Hello, world!', 'utf8').toString('base64url')); // works
//> 'SGVsbG8sIHdvcmxkIQ'

console.log(BrowserBuffer.from('Hello, world!', 'utf8').toString('base64url')); // fails
```

**Error:**

```
Uncaught TypeError: Unknown encoding: base64url
slowToString index.js:544
toString index.js:606
encode +page.svelte:41
debugger eval code:1
[index.js:544:31](http://localhost:5173/node_modules/buffer/index.js)
slowToString index.js:544
toString index.js:606
encode +page.svelte:41
debugger eval code:1
```

This may break some stuff, so should be fixed soon!