Add completions for linux packages

cmd/sing-box/generate_completions.go

@@ -0,0 +1,28 @@
+//go:build generate && generate_completions
+
+package main
+
+import "github.com/sagernet/sing-box/log"
+
+func main() {
+	err := generateCompletions()
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func generateCompletions() error {
+	err := mainCommand.GenBashCompletionFile("release/completions/sing-box.bash")
+	if err != nil {
+		return err
+	}
+	err = mainCommand.GenFishCompletionFile("release/completions/sing-box.fish", true)
+	if err != nil {
+		return err
+	}
+	err = mainCommand.GenZshCompletionFile("release/completions/sing-box.zsh")
+	if err != nil {
+		return err
+	}
+	return nil
+}