[Feature Request] Fish completions

I have written some basic fish completions for your CLI, which i'm quite happy with and i would like to provide them to include them directly into your package. https://gist.github.com/dafitt/4aec26d1107c1a9b1d9812d9eb6dc9b2 This file would be installed to `share/fish/vendor_completions.d/uwsm.fish`. See https://github.com/fish-shell/fish-shell/blob/master/doc_src/completions.rst#where-to-put-completions.