Update progress notifier & add example tests

[camilledejoye]

This integrates the changes proposed in phpactor/language-server#39
And add the test I forgot the last time, which can be a replacement for #24 once the new interface for handling progress notification will be validated.

Next stop:

• Integrating it in the index handler in phpactor LS extension
• Integrating it in the references handler in phpactor LS extension
  • I already did a first try with the previous implementation but I don't know any client which specify the workDoneToken so I add to create my own implementation to test it on my machine... Does CoC handles it (it would be a pain to install it again but I can try)