Support relative links via @-references

Ideally, users would be able to express relative links not just via `#link` but also via Typst's `@`-references. For example:

```typst
// intro.typ

# Introduction <intro>

This is an introduction.
```

```typst
// conclusion.typ

As described in @intro, we have shown...
```

Today, the user would manually substitute something like `#link("./intro.typ#intro", [Section 1])`.
