support configurable line separators in StackTraceFormatter

There are use cases where it is desirable to avoid the use of platform line separators when formatting stack traces for log output:

https://github.com/quarkusio/quarkus/issues/6262

Since the StackTraceFormatter itself already has an abstraction for new line handling, it would be trivial to support the use of an alternate string:

https://github.com/jboss-logging/jboss-logmanager/blob/main/core/src/main/java/org/jboss/logmanager/formatters/StackTraceFormatter.java#L160

The non-trivial part is coming up with a way to specify the alternate string. An obvious choice would be to use the argument section in the format specifier, but this could be cumbersome given that the argument is already used for indicating suppressedDepth. and users will want to specify control characters or literal escape-sequences. Perhaps a simple alternative would be to use a system property?