Add WP-CLI Shell Completion

[timnolte]

For Dev Container productivity it would be helpful to add WP-CLI completion. This should support both Bash & Zsh.

Source: https://raw.githubusercontent.com/wp-cli/wp-cli/main/utils/wp-completion.bash

See: https://github.com/timnolte/.shell/blob/main/.config/dev-shell/setup.sh#L73C2-L73C2