Support for terraform -install-autocomplete bash

[shellwhale]

Currently terraform-target-autocompletion overwrites the bash script generated by terraform -install-autocomplete bash. We need to find a way to make them compatible.

[pataquets]

A widely found practice is for the command to provide some completion (bash|zsh|fish|...) kind of subcommand which dumps the corresponding autocomplete shell script, just ready to evaluate it as "$(yourcommand completion bash)". Tools like gh, glab, rclone do it this way, to name a few.