basePath not taking a leading slash

Suggested fix https://github.com/cloudflare/partykit/compare/main...nachobibian:partykit:patch-1

```javascript
// if user provided a leading slash, remove it
if (basePath?.startsWith("/")) {
basePath = basePath.slice(1);
}
```

Start by searching the TypeScript source for basePath handling and compare the current behavior with the suggested change in the issue body. Verify that a basePath beginning with a slash is accepted and normalized as intended; done means the reported leading-slash case works without regressing the existing behavior.

Written by the indexing model from the issue text.