Switch from MSBuild properties to an AdditionalFiles json file

[AArnott]

This allows unity (which has no msbuild properties) to configure the source generator via a JSON file passed to the compiler via AdditionalFiles.

Closes #1650

[neuecc]

It's wonderful!

Would the URL for the example schema in the ReadMe be
https://raw.githubusercontent.com/MessagePack-CSharp/MessagePack-CSharp/master/MessagePackAnalyzer.schema.json ?
Also, I think it's fine that the schema.json serves as the document, but it might be nice to have a link to it.

[AArnott]

Would the URL for the example schema in the ReadMe be
https://raw.githubusercontent.com/MessagePack-CSharp/MessagePack-CSharp/master/MessagePackAnalyzer.schema.json ?

Yes, I should have used MessagePack-CSharp as the account name. I'll fix that.
As for master instead of develop, that would delay its actual correctness to when develop finally merges into master (could be months away). So at least for now I think develop is better so folks can follow the README and get a working schema.

Also, I think it's fine that the schema.json serves as the document, but it might be nice to have a link to it.

I don't understand. Can you explain more about what you mean?

[neuecc]

So at least for now I think develop is better so folks can follow the README and get a working schema.

OK!

Can you explain more about what you mean?

For example

The above example mostly sets defaults. You can discover all the available settings in MessagePackAnalyzer.schema.json, their defaults and meanings in a JSON editor that supports JSON schema.