Revisit parse_string/2 return contract for 0.4.0

[jeffhuen]

RustyCSV.parse_string/2 currently has an option-dependent return shape:

• default: returns rows
• headers: true: returns maps
• input too large: returns {:error, :input_too_large}
• invalid options: raises ArgumentError

This is intentional for compatibility and ergonomics, but it matches Elixir's "alternative return types" design anti-pattern and makes the public contract harder to reason about.

For 0.4.0, consider introducing explicit APIs with stable return shapes, for example:

• parse_rows_string/2
• parse_maps_string/2
• parse_string!/2
• parse_maps_string!/2

Then decide whether to deprecate or change the headers: option behavior.

This should not be changed in a 0.3.x patch release because changing parse_string/2 directly would be breaking.