Add `sidePanel` API

[fregante]

See https://developer.chrome.com/docs/extensions/reference/api/sidePanel

[RickyMarou]

Hello - I couldn't transfer the repository so I had to re-create it and lost the open issues. Would you mind re-opening the issue in the new home of the project here: https://github.com/RickyMarou/jest-webextension-mock/issues

Some quick thoughts:

• I think the API is small enough that I can implement it, i'll also happily accept a PR for it.
• I'd like to understand better the difference between the chrome sidepanel API and the firefox sidebarAction 🤔
• This library has historically been supporting real world implementation of the web extension API instead of strictly sticking to the specs, which makes total sense and want to continue doing, so it's a good argument for adding the sidePanel API even tho it's chrome only.