readline: improve unicode support and tab completion

1. This reduces the number of write operations used during tab
   completion.
2. The tab completion calculated the string width using the length
   of the string instead of using the actual width. That is fixed.
3. The key decoder is now capable of handling characters composed
   out of two code points. That reduces the number of "keypress"
   events that are emitted which again lowers the amount of writes
   triggered.

PR-URL: https://github.com/nodejs/node/pull/31288


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>