There's nothing in the way of classes specifically for WME. If you search the Wiki for "scripts" you will find a list of scripts plus a link to some basic starter information.

The best way to get started is simply to examine some of the scripts that already exist. Apart from a couple of the most powerful scripts which have protections to hide the code, the source code for all the scripts is easily available. Have a look through some and see what they are doing and how.

The other advice I would give is "dive in". Create a basic framework script that does maybe nothing more than create a tab in the LH pane. Then experiment to see what you can do. For my own script development, I use a decent text-editor and edit the script directly in the Chrome addons folder. That lets me make changes and see what happens immediately on refresh - but I do have to be careful about keeping backup copies in case I make a mess of it.