Consider renaming field option default to fallback

[schickling]

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.

[stale]

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