make the base of URL constructor configurable?

I have a rather niche use case (and will probably deploy a fork for now) where I'm trying to use HTMX out of an iframe with `[href=about:srcdoc]`. So when HTMX tries to construct any URL at all it's expanded to `new URL("the path", "about:srcdoc")` which obviously isn't a valid url.

Would it be possible to add a config option to specify the base parameter?

Like
```typescript
urlBase?: undefined | string | "root"
```

where `"root"` would have the special meaning of using the root windows url.