add Thorium to `web-browser-extension-distribution-information.json`

[rhysdh540]

I'd like to add support for the Thorium browser to the Web Browser Extension Distribution Information file. This would resolve issues with the iCloud Passwords extension (see https://github.com/Alex313031/Thorium-MacOS/issues/26, https://github.com/Alex313031/Thorium-MacOS/issues/68).

One potential challenge is that Thorium is currently ad-hoc signed, and the developers are unwilling to obtain a developer id. This may affect what can be provided for the code_signing_team_identifier field, but I wanted to raise the request for consideration in case there are possible accommodations or future updates that could allow inclusion.