Sveltia config: `singletons` not supported

[marcuspollio]

Sveltia 0.83 introduced the singletons feature. In short, this allows to directly manage single files, instead of using nested file collections.

Currently, the Headless-cms Hugo module (v0.1.2) does not support singletons. A PR is pending.

[privatemaker]

@marcuspollio hi there, thank you for this issue and pull-request. My apologies for not responding. I was on holiday and my head was in other codebases. I need to explore the Sveltia singletons and test this but it should be a nice addition 😄