docs(configuration): add onListening, update after and before in devServer

[billy-le]

onListening was added in 3.5.0
webpack/webpack-dev-server#1760

compiler arg for after and before functions was added in 3.3.0
https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md#features-4

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[EugeneHlushko]

Thank you, although i have to request changes on the example and the wording

[EugeneHlushko]

This isn't a great example, can we change this to:

Suggested change

	// do fancy stuff
	const port = server.listeningApp.address().port;
	console.log('Listening on port:', port);

[EugeneHlushko]

Suggested change

	This option allow to developers run own scripts after run webpack-dev-server
	Provides an option to execute a custom function when `webpack-dev-server` starts listening for connections on a port.

[billy-le]

Thank you for the feedback! I've updated the PR as requested.

[EugeneHlushko]

Thanks!