Add getLogger() method to pipeline object
Spun off from #799 (closed), the pipeline object that is passed to the register function for Antora extensions and bound to the event listeners should provide a getLogger()
method that wraps the following logic:
pipeline.require('@antora/logger')('<name>')
thus simplifying the call to:
pipeline.getLogger('<name>')
An open question is what the behavior should be if a name is not specified. Should it default to 'antora', 'antora-extension', or should it return the root logger (which is nameless). I'm leaning towards 'antora' (or even 'extension'). As a general practice, the user should specify a name.