Enable globally turning off warnings about using directives in multiple files

[Gedochao]

We should add a global configuration enabling users to silence the warning about migrating directives to project.scala. It should be settable with the config sub-command.

So something like:

scala-cli config ignore-directives-in-multiple-files true

Ticket motivated by a comment by @lbialy in #1632 (comment)

[SethTisue]

what am I doing wrong...?

% scala-cli --version
Scala CLI version: 0.2.0
Scala version (default): 3.2.2
% scala-cli config ignore-directives-in-multiple-files true
Error: unrecognized key ignore-directives-in-multiple-files

[tgodzik]

It should be suppress-warning.directives-in-multiple-files