Consider renaming field option `default` to `fallback`

I'm currently considering renaming the field option `default` to `fallback` within the `defineDocumentType` or `defineNestedType` definitions in `contentlayer/source-files`.

Reasoning:
The term `default` value is typically used when "creating" some kind of record - however, Contentlayer never creates any kind of records/documents but just transforms then. Therefore I'm arguing the term `fallback` is more appropriate when considering the **absence** of a value.