invalid type: map, expected a sequence with []

[Myriad-Dreamin]

Some early users reported a hard-to-guess error:

typst-book error: convert_to<BookMeta>: invalid type: map, expected a sequence with []

This is because we (and typst compiler) don't check type for book meta fields. This error should be more readable after we do some manual type checking in the @preview/book package.