[vi-mode] Support diw command

[springcomp]

Description of the new feature/enhancement

As alluded to in #584, PSRL should support vim "text-objects".
Consider supporting the diw command to delete inner words.

Proposed technical implementation details (optional)

I think a new Chord dispatch table must be introduced because currently, PSRL cannot handle 3-letter commands.
Most of the implementation from ProcessOneKey can be reused and refactored in common methods.