Shell completion support on a new level

[lefou]

If would be great to have some built-in shell completion support.

Steps to realize in Mill:

• a target to generate a file with all completable targets
• some logic in Mill server, to always generate that file (if outdated) after the server becomes idle

Of course, existing shell completion tools need to use this generated file. They also need a fast way to check whether this file is up-to-date, so they can trigger a re-generation.