Add dynamic shell completion for --profile and --region

[huseyinbabal]

Summary

• Add custom completion module with scripts for bash, zsh, fish, and PowerShell
• Add hidden list-profiles and list-regions subcommands used by completion scripts
• Completions dynamically fetch profiles from ~/.aws/config and ~/.aws/credentials

Usage

# Add to ~/.bashrc (or ~/.zshrc for zsh):
eval "$(taws completion bash)"

# Then use:
taws --profile <TAB>  # Shows: default, production, staging, etc.
taws --region <TAB>   # Shows: us-east-1, us-west-2, eu-west-1, etc.

Supported Shells

• Bash: Full dynamic completion
• Zsh: Full dynamic completion with descriptions
• Fish: Full dynamic completion
• PowerShell: Full dynamic completion
• Elvish: Falls back to clap's default static completion

Closes #97