Add built-in support for CSV/TSV

Use case:

Excel/Google sheet worksheets saved as CSV/TSV

- Use https://www.npmjs.com/package/csv-parse as optional peerDependency (similar to toml implementation)
- CSV can be parsed either as array of arrays or array of objects. A CSV is not guaranteed to have a header row so array of arrays is safer.
- will require being able to specify options at some point (breaking change with current)

Start by tracing the existing TOML implementation and reviewing how optional peer dependencies are handled. Use csv-parse to support CSV and TSV input, with array-of-arrays output as the safe default; done means both formats load worksheet data without requiring the dependency unless this feature is used. Options remain an unresolved design concern in the issue.

Written by the indexing model from the issue text.