The introduction of custom formats has caused an additional regression on partial completions.

Minimal reproduction:

```
---
format: 
  html:
    c_ # <- complete from cursor in "_"
---
```

yields a different completion set from:

```
---
format: 
  html:
    _ # <- complete from cursor in "_"
---
```

The issue is we're confusing the context in which partial completions should be allowed.
cscheid
2022-07-18T16:49:20Z
cscheid
yaml-validation editors
closed
Partial yaml completion regression
https://github.com/quarto-dev/quarto-cli/issues/1448