[Tech] Move to pnpm

[CommandMC]

This involved

• specifying our Electron dependency properly (we were missing the "github:" protocol)
• adding some missing '@types' packages (and in turn fixing some type issues that weren't caught before)
• fixing imports from 'react-router' (it's 'react-router-dom')
• installing pnpm in Workflow runs
• "Find and replace"ing quite a few "yarn"s with "pnpm"s

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[flavioislima]

Tested here and is working fine.
would just be good to double check if AppVeyor has pnpm installed on their docker images already, otherwise the windows release will fail.

[flavioislima]

Perhaps lets keep using npm or yarn on AppVeyor since it is just one of the builds and all the rest is working fine.

[CommandMC]

Using something other than pnpm would still leave us without a lock file, but I guess that's better than it not working at all

[CommandMC]