Suggestion: Add "lib" reference directive

[rbuckton]

Following discussion at DefinitelyTyped/DefinitelyTyped#15140, it seems that we might benefit from adding a new lib reference directive similar to the types reference directive we have today. The lib reference directive would allow an individual file to specify one or more lib files to include when the file is included.

Example

/// <reference lib="es2015, dom" />

This would allow individuals using @types/core-js (or any other polyfill/shim library) to automatically get the correct types.