Add signing certificate for Windows installer

[JWCook]

Currently, the Windows installer triggers a warning from Microsoft Defender SmartScreen ("SmartScreen prevented an unrecognized app from starting...").

To get rid of this, the app needs to be code-signed with a trusted certificate. Those tend to be expensive, but SignPath offers free code signing for some open source projects. There is an application process, but this project may meet the requirements (except evidence of popularity...?). Terms of use: https://signpath.org/terms

[JWCook]

#452 Is related: A paid certificate would be required to avoid quarantining by macOS Gatekeeper. There is very little chance of me paying $100/year for that, though.