usage: add shell completions

See https://usage.jdx.dev/cli/completions#completions-for-usage-cli-itself.

Author: muzimuzhi

Formula/u/usage.rb

@@ -19,6 +19,7 @@ class Usage < Formula
 
   def install
     system "cargo", "install", *std_cargo_args(path: "cli")
+    generate_completions_from_executable(bin/"usage", "--completions")
   end
 
   test do