JSII config file

[jci-aws]

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). Type-checking the config would also be a nice-to-have here.