Add support for setting the basePath on line formatter

[Seldaek]

See https://github.com/Seldaek/monolog/releases/tag/3.6.0 / Seldaek/monolog#1873

It would be neat I think if we could do base_path: %kernel.project_dir% on a handler and it would configure the line formatter for it with base path set.

It could work like the include_stacktraces functionality I'd say.