Add auto complete for sub commands

[maxhartke]

The polymarket CLI has no shell completion support, making it harder to discover subcommands and flags interactively. Tools like docker, gh, and kubectl all ship with completion scripts (zsh, bash, fish) or support a completion subcommand that generates them on demand. It would be great if polymarket either bundled completion scripts or supported polymarket completion zsh|bash|fish — the latter is easy to add with clap's built-in clap_complete crate and takes minimal effort. Happy to contribute a PR if that would help.

[mvanhorn]

I've submitted a fix for this in PR #40. Uses clap_complete to add a polymarket completions <shell> subcommand supporting bash, zsh, fish, elvish, and PowerShell.