Make Mill commands require named CLI parameters by default

[lihaoyi]

This aligns the semantics with MainArgs library defaults. Previously we grandfathered in the "positional by default" behavior because Mill pre-dates mainargs, with original behavior inherited from Ammonite. But most CLIs do not allow keyword args to be naively replaced by positional args, and MainArgs thus chose to have such behavior as opt-in. Mill should follow suite

Added a flag --allow-positional-command-args to fall back to the old behavior, and covered this behavior with example tests, docs, and updated the changelog.

This is a binary-compatible but semantically-incompatible change, so it should go into 0.12.0. It's a pretty painful breaking change, but it's definitely the right thing to do long term, so better to get it over with. People can also add --allow-positional-command-args to their ./mill file if they want to preserve the old behavior in perpetuity

[lefou]

Looks like a good candidate for persistent local configuration.

• Mill config file concept (RFC) #1226