Support smithy-build.json

[srchase]

Add support for syntax and diagnostics of the smithy-build.json file, including surfacing errors related to the configuration of plugins, projections, transforms, or environmental variables defined in smithy-build.json.

Auto-fix suggestions would be helpful as well.

[kubukoz]

A quick win here might be to register the JSON Schema of the file format. This would automatically add support for (at least) the basic structure of the file.