Add fish suport to installer

Before this patch, installing Nix using the Fish shell did not
work because Fish wasn't configured to add Nix to the PATH. Some
options in #1512 offered workarounds, but they typically involve
extra plugins or packages.

This patch adds native, out-of-the-box support for the Fish shell.

Note that Fish supports a `conf.d` directory, which is intended
for exactly use cases like this: software projects distributing
shell snippets. This patch takes advantage of it. The installer
doesn't append any Nix loader behavior to any Fish config file.
Because of that, the uninstall process is smooth and a reinstall
obliterates the existing nix.fish files that we place instead of
bothering the user with a backup / manual removal.

Both single-user and multi-user cases are covered. It has been
tested on Ubuntu, and a Mac with MacPorts, homebrew, and the
Fish installer pkg.

Closes #1512

Co-authored-by: Graham Christensen <graham@grahamc.com>

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