feat: support additional history filenames in replxx importer

[waldyrious]

The original implementation of the replxx importer, from PR #2024, only supported .histfile as a filename for the history file, since there is no default filename. However, the replxx codebase does use replxx_history.txt as an example history filename (see here and here), so this patch adds support for that as an alternative filename.

The implementation was modeled after the one currently used for the zsh history file importer, which also means the replxx importer now produces a human-friendly error if no history file is found in the expected path(s).

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

/cc @amosbird who introduced this importer in #2024.

[ellie]

looks good to me, thank you!