Fix reverse import order for zsh and nu

[ajesipow]

Should fix #178

Contains the following changes:

1. Added DoubleEndedIterator implementation for UnixByteLines.

2. Reversed import order for zsh
   I had to make some additional changes to handle commands spanning multiple lines: the previous command is prepended to the current one if it contains a line break (i.e. we need to keep the original import order for commands on multiple lines).

3. Reversed import order for nu
   Simply reversed the order for nu. Commands spanning multiple lines do not need to be handled differently since they are stored in a single line in the history.

The import order for bash, fish and replxx seem to be correct already.
I also did not touch resh as I could not test locally since it tries importing ".resh_history.json" whereas resh seems to be storing its history in history.reshjson. So that appears to be a separate issue unless I missed something.

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing