Provides SourceGenerator option from AdditionalFiles #1650

Source Generator in Unity, can not provide option from AnalyzerConfigOptions so can not configure any options.
If you want to deprecated mpc (which I agree with), we must provide configuration for Unity.

MemoryPack has made that mistake and is currently unable to provide the option from Unity, despite requests.
Cysharp/MemoryPack#133

However, Unity supports AdditionalFiles.
Usage differs from the standard .NET, probably in two ways.

• Edit -> Project Settings -> Player has additional compiler arguments, and enter /additionalfiles:xxx
• ScriptCompilerOptions.RoslynAdditionalFilePaths

My suggestion would be to allow the settings to be described in JSON and include that in Parse as well.