Add a shell completions command

Here is an example of other tools that support this:

uv generate-shell-completion fish | source
bat --completion fish | source
rg --generate complete-fish | source
fd --gen-completions fish | source
hexyl --completion fish | source

The rationale is when installing hyperfine via tools like mise or aqua, we need a way to source the completions. One way would be if these "tool/binary managers" would bundle the completions in this lib.

But having a command to generate them would be better for cases where the default completions have not been bundled.

I am working around this now like this with mise, but it is not as nice and idiomatic as having a command that can generate them:

cp "$(mise where hyperfine)"/hyperfine-*/autocomplete/hyperfine.fish ~/.config/fish/completions/hyperfine.fish