Add a helper to compute a path for a quarto project root

[cderv]

Related to discussion at quarto-dev/quarto-cli#9649

Project root should be accessible in QUARTO_PROJECT_ROOT env var. So we could use this information to build a path

Something like

```{r}
#| label: embedme
readLines(xfun::from_root('data.txt', root = Sys.getenv("QUARTO_PROJECT_DIR")))
```

Though maybe this is just documentation recipe topic has latest here 📦 has a new mechanism quite simple

```{r}
#| include: false
here::i_am("assets/embed.qmd")
```

```{r}
#| label: embedme
readLines(here::here('data.txt'))
```