Generate a programmatically-consumable, typesafe API for an external bundle

[afoxman]

An external bundle exports a collection of packages, each under a unique name. The external bundle author specifies this mapping as kit configuration.

For example, it might look like this:

"rnx-kit": {
    "externalBundle": {
        "interface": {
            "MyKitGlobal.Telemetry": "telemetryjs",
            "MyKitGlobal.StringResources": "string-resources",
            "MyKitGlobal.ReactNative.Components": "react-native"
        }
    }
}

The kit bundler will take this configuration, and use it to generate a programmatically-consumable, typesafe API. Something along the lines of a typescript .d.ts file.

Other packages which depend on this kit will naturally gain access to the typesafe API, and benefit from it in IDEs like VSCode and at lint/build time.

[afoxman]

Merged into #1283