feat: add option to generate TypeScript project references (#343)

TypeScript project references bring down the compilation times of TypeScript projects
in a monorepo (by avoiding repeated recompilation of the same files over and over again).

They also allow a convenient "full rebuild" of the entire repository in a couple of seconds by running `tsc -b`.

This change also introduces a cache when loading dependency JSII assemblies, so that repeated use of the same assembly isn't constantly being loaded and validated.

Both of these changes bring compile times down drastically on the CDK repository (on my machine, a full rebuild goes from 16m down to 3m30).

ALSO IN THIS CHANGE

* Change webpack-command => webpack-cli, since the older one is deprecated (see https://github.com/webpack-contrib/webpack-command) and was giving me the following error: "TypeError: Path must be a string. Received undefined". Upgrading to webpack-cli fixed it.