4.x initial PR

[oshai]

Initial 4.x version.
Notable changes:

• Change root package from mu to io.github.oshai (to allow gradual migration to new api).
• Change module names to kotlin-logging.
• Change most of jvm api's not to depend on slf4j.
• Change slf4j dependency to compileOnly (so slf4j users need to add it at runtime).
• Move slf4j specific methods to Slf4jExtensions.kt
• Add JUL impl. configured by -Dkotlin-logging-to-jul

[oshai]

Continue in #269