Support LogBuilder API in a kotlin-y way

The Kotlin API does not have an equivalent to the https://logging.apache.org/log4j/2.x/manual/logbuilder.html.

A Kotlin-y approach may use a DSL rather than a builder. For example, a `LoggingScope` passed as a lambda receiver could result in an API that could look something like:

```kotlin
logger.atError {
// `this` here is an instance of `LoggingScope`
// everything optional except log
throwable = ...
marker = ...
location = ...
log = ...
}
```

The standard builder API could also be supported as an alternative for users who prefer it.

See also relevant background discussion about designing a fluent logging API from https://github.com/google/flogger and https://google.github.io/flogger/benefits.

Start by reviewing the existing Kotlin API, the linked Log4j LogBuilder documentation, and the Flogger design discussion. Done means agreeing on and implementing a Kotlin-friendly DSL or builder that provides equivalent LogBuilder support, including optional throwable, marker, location, and log values.

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