Add `programs.nushell.abbreviations`, mirroring `shellAliases`, to
declaratively configure fish-style abbreviations (nushell 0.113.0+).
The set is emitted as `$env.config.abbreviations = { ... }` after the
aliases so it survives a wholesale `$env.config = { ... }` written in
`extraConfig`.
Unlike aliases, abbreviations expand inline in the line editor, so the
expanded command is what ends up in history.
