set QUARTO_PROJECT_ROOT to root of quarto project

[cscheid]

This would allow

```{python}
from pathlib import Path
from os import getenv
import pandas as pd
df = pd.read_csv(Path(os.getenv("QUARTO_PROJECT_ROOT")) / "docs/computations/palmer-penguins.csv"))
```

The idea is to make referring to project-wide assets in computational content as convenient as a project-relative path in markdown, such as /docs/assets/logo.png.

[cscheid]

the Path Python class is designed to be extensible. We could envision a quarto_cli Python package providing a QuartoProjectPath class that does this kind of path resolution.