Add basic bash completion script

[brmzkw]

Initial version generated with ChatGPT/o3-mini-high.

It uses plakar -h and plakar -h output, so positional arguments aren’t handled well.

For example, completion for commands like “plakar on” or “plakar ls xxxx:” is suboptimal. Only bash is supported for now.

Note this is a draft PR, we might want to put the script somewhere else. Any idea?