Produce correct relative URLs

### What is the problem this feature will solve?

The URL Standard describes how to combine a string with a relative path with a base URL to produce a valid absolute URL.

This feature request is: given an absolute URL and a base URL, produce a string with a relative path that when joined to the base URL, would produce the original absolute URL.

This is similar to file system path operations like Go's [Rel](https://pkg.go.dev/path/filepath#Rel) and Python's [relpath](https://docs.python.org/3/library/os.path.html#os.path.relpath).

Practical application: some HLS video players expect relative paths, despite the HLS spec allowing for absolute paths. I want to produce correct relative paths given the relevant absolute ones.

### What is the feature you are proposing to solve the problem?

Some function like:

```
ada_relative_path(base, input)
```

Example:
* base: `https://example.org:8080/origin/multivariant/playlist.m3u8`
* input: `https://example.org:8080/origin/media/1080p.m3u8`
* output: `../media/1080p.m3u8`
* validate: parse output with base, get back input

### What alternatives have you considered?

Using the file system tools.