Detecting addons that are already installed

[khlam]

Suppose a user wants to switch to WoA but they already have addons installed (Possibly due to previously using Curse/Twitch).
As of WoA 1.0.0, there is no mechanism to detect addons that are already installed.
Any previously installed "untracked" addons will not be in addons.json and WoA will not be able to update or manage them (WoA will be able to detect them thanks to chokidar, but it can't do anything to them).

This story shall be considered complete when:

• WoA detects what addons are installed but are not in addons.json.
• If an addon is hosted on multiple sites (curseforge, wowinterface, github) then WoA will offer a choice of where to install from.
• This system is secure from tampering so there is credit to addon creators (incorporate donation/patreon links), transparent so people can trust there is no malware, and decentralized so that no site/host can monopolize addon distribution,