Removes the Linux guard (was: exit 1 with apt suggestion).
Adds pkg_install branch for Linux: extracts bare fish binary from
fish-VERSION-linux-ARCH.tar.xz into ~/.local/opt/fish-vVERSION/bin/.
Adds _linux_post_install() with chsh instructions.
macOS: installs fish*.app to /Applications/fish.app (covers v3 fish.app
and v4 fish-VERSION.app); pkg_link symlinks Contents/MacOS/fish.
OS detection via uname -s at top level; functions have no OS guards.
pkg_link avoids calling webi_link (would recurse since webi_link calls
pkg_link when defined).
Replaces [ with test throughout per POSIX shell conventions.
Co-authored-by: AJ ONeal <aj@therootcompany.com>
