provide relative path argument to invocation_directory (Feature Request)

In lieu of https://github.com/casey/just/issues/1159, I'd like to request that invocation_directory take an optional argument, resolving it relative to the invocation directory. I'd also like to request that it resolves the path completely.

For example, instead of:

```
absolute_path(join(invocation_directory(),'../here'))
```

One could instead write:

```
invocation_directory('../here')
```

Start by locating the implementation and tests for the invocation_directory function. Check how its current no-argument form resolves the invocation directory, then trace the existing path-resolution behavior. Done means an optional relative argument is accepted, resolved against the invocation directory, and fully resolved as shown by the issue's example.

Written by the indexing model from the issue text.