feat: add Prefix, a code cut off after a segment

[Adeniyikayodee]

A prefix is a postcode cut off after one of its segments, such as the district EK-01-A03. It names every code that starts with it, so it is the value to group or select records by: state, LGA, district, or area. Until now the libraries only produced a prefix as a string.

• Spec first: spec/vectors.json gains cases for parsing a prefix, for which codes it contains, and for its parent. All four languages run them.
• The same surface in each language: parse a prefix in any style, read through (its last segment), take its parent, ask whether it contains a code, and cut a code down with truncate. A whole code is the narrowest prefix.
• truncate as the name: it mirrors date_trunc in SQL and dt.truncate in dataframes, which is how the people who will group by place already think. The existing prefix() keeps returning a string, since changing it would quietly break comparisons in Python and JavaScript.
• One new error: a prefix must end where a segment does, after 2, 4, 7, 9, or 11 characters. Anything else is a prefix length error with the same message everywhere.

Nothing is released by this. Java will need 0.3.0 when it is, because the new error is a case on the sealed ParseError, which breaks an exhaustive switch in a caller's code. Rust's enum is non-exhaustive, so the crate does not break.