Re-design Logger hierarchy

[lihaoyi]

The Logger interface is a bit of a mess, doing a bunch of different things some Mill-internal and some user-facing. #3577 extends it a bit but makes it even more messy.

Probably we should have more narrow specialized Logger interfaces for different parts of the codebase, e.g. the logger inside a Task would have a ticker, whereas the logger outside would not, and a top-level logger would have more power to do things like configuring the prompt that task-specific loggers would not have. close() probably needs to be re-thought since the lifecycle of the various loggers is all different, with some short-lived and some long-live

The whole color scheme and debug/info/warning/error conventions also need to be redone. Currently infoColor is blue and normally used for "less important" things, but then logger.info uses it for normal-important things. And how should a user decide between info and just println? When should subclasses of Logger put logic into overriding info vs injecting logic into stdout/stderr streams?

Arguably, all the concrete FooLogger classes should be private[mill], since there's no reason that a Mill user would need to instantiate their own PrintLogger or whatever.

This would break bincompat and would have to go into 0.13.0

[lihaoyi]

At time of writing this is the current logging hierarchy that is typically available inside any Mill task:

MultiLogger(
    PrefixLogger(
        PrefixLogger(
            PromptLogger("foo.test"),
            "",
            ""
        ),
        "[106] ",
        ""
    ),
    FileLogger(/Users/lihaoyi/Github/mill/example/scalalib/testing/4-test-sharding/out/foo/test/test.log)
)