This moves YAML formatting behind an unstable flag for Deno 1.46. This
will make it opt-in to start and then we can remove the flag to make it
on by default in version of Deno after that.
This can be specified by doing `deno fmt --unstable-yaml` or by
specifying the following in a deno.json file:
```json
{
"unstable": ["fmt-yaml"]
}
```
