Enable JSON schema for appsettings.json

[JamesNK]

appsettings.json is the built-in file name for providing configuration in .NET apps. I'm currently improving the schema for appsettings.json on schema store.

Visual Studio 2019 automatically selects a schema from schema store for appsettings.json (and appsettings.{configuration}.json) files:

VS code should also automatically use it as the schema for those files.

I believe an extension can specify the schema for a file using this API:

https://code.visualstudio.com/api/references/contribution-points#contributes.jsonValidation