Make `tags` optional when formatting

[artemisSystem]

Would be nice if you could leave out the tags field when formatting if you don't need to format type tags. Currently it just errors if it's missing.

[larsgw]

This is probably a bit annoying with TypeScript. When parsing a document it's nice to have a guarantee that node.values is always an array, for example. So it would need a separate set of types for the formatter, I think.