LIBRARY - implement API for Etherpad v1.8.18, add return and nullable types

[codeofmochi]

Hi there!

Thank you for your very useful library! I've been forking it for our own purposes at @graasp 😊
I'd like to contribute back with the following improvements:

• lockfile upgraded to v2 (generated using npm 8.3.2) 🔒
• build the package on prepare script, so that it can be consumed from JS code as well 🏗️
• remove dependency on is-url and use the native URL node object instead (reason is that is-url does not accept e.g. docker-local domains such as http://etherpad:9001) and it can be checked without a library 🌍
• implement the API described at the etherpad v1.8.18 docs 📝
• add nullable types withstrictuNullChecks: true 👮‍♀️
• return 400 instead of 422 on bad request 🚫
• add queries return types (derived from existing types) 🐚
• add a deprecated notice to setPassword and isPasswordProtected (they are now absent from the docs) 👴
• add missing getStats query 📊 as well as copyPadWithoutHistory ✂️

Somehow I am not able to make the server error - timeout pass in the test suite even though nock is appended .delay(100) on the getHtml endpoint for this test, but I think this was an issue already before these changes.

Let me know if you'd like additional changes! Feel free to squash the commits as well.

Cheers