Improve fish binding

[mingmingrr]

Fish 3.1 added $pipestatus, which should be preferable over $status in --last-pipe-status.
Fish 3.0 added $fish_pid, which should be preferable over (random) in --renderer_arg=client_id.
Older versions of fish can fall back on $status and (random).

%self is also available instead of $fish_pid prior to fish 3.0, but I've opted not to use it due to process expansion having been removed at some point, and there being no good way of checking whether %self is usable.

Additionally, test -z/-n and env ara replaced with fish builtins set -q eval where applicable.

Manually tested on NixOS / fish 3.3.1 / tmux 3.2a and Ubuntu / fish 2.1.2 / tmux 3.0a