Hi and first thanks for this awesome project.

I have a problem since version [0.2.232](https://github.com/quarto-dev/quarto-cli/commit/fd6d8cd9709c23f38bc76617fab29fc578358033) introduced an error on my metadata parsing:

```markdown
---
matter: "NSI"
classe: "Terminale"
partie: "P1&P4"
sectionPrefix: "Partie~"
subsectionPrefix: "Exercice~"
format:
  latex:
    papersize: a5
    classoption:
      - fontsize=10pt
      - DIV=calc
---

# Devoir 3
```

When i run quarto I go onto this:

```
ERROR: YAMLError: can not read a block mapping entry; a multiline key may not be an implicit key at line 6, column 7:
    format:
```

However this is valid `yaml` and pandoc can parse it. `pandoc -t json devoir3.qmd | jq '.meta' `.


2021-11-23T14:51:17Z
benabel
closed
quarto can't parse my metadata since v2.232 
https://github.com/quarto-dev/quarto-cli/issues/198