Chord keybindings: one shared resolver over a trie

#989 and #1016 went after multi-key bindings so the combined design lives here.

• Storage: a trie in Keybindings keyed by KeyCombination, as in #989.
  A key may be both a binding and a prefix (j -> Down, jj -> Esc).
• Recogniser: one SequenceResolver in edit_mode that each mode embeds, as in #1016 but written once.
  The mode only supplies the fallback, e.g. vi insert self-inserts a lone j once the sequence dies.
• Timeout: the pending sequence flushes on the engine side, since only the engine sees time pass.
• Tests: through the vi harness, an emacs counterpart, and the four cases: completes, times out, strict prefix, binding-and-prefix.

Config shape on the nushell side and f<char> style capture are follow-ups.

Thanks @reubeno and @benvansleen for the two halves.

Start by inspecting Keybindings and the edit_mode entry points, then trace how the vi harness currently handles key sequences. Verify the shared resolver against the vi harness, an emacs counterpart, and the four stated cases: completion, timeout, strict prefix, and a binding that is also a prefix.