feat: add option to output document as raw string

[Hopsken]

Add a raw option to content resolve function so it will output raw string instead parsed AST.

This could be useful when user need to fetch the collection entry in the client side since markdoc AST is not serializable.

But I need help about adjusting the type declaration, the generic here is pure MAGIC.

[changeset-bot]

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Latest commit: d58aaf8

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