JSII config file

It would be nice to define JSII configuration in a separate configuration file, instead of cluttering `package.json` with JSII-specific configuration. Something like `.jsii`, `.jsii.yaml`, `jsii.config.ts`, etc.

All JSII configuration that currently exists in the `package.json` `jsii` field should be able to be specified here (including [diagnostics](https://aws.github.io/jsii/user-guides/lib-author/configuration/#diagnostics)). Type-checking the config would also be a nice-to-have here.

Start by tracing how the `jsii` field in `package.json` is read and how the linked diagnostics configuration is handled. Define a separate configuration format that can express all existing JSII settings, including diagnostics; completion would mean equivalent configuration works outside `package.json`, with type-checking if supported.

Written by the indexing model from the issue text.