Add guide for using google-api-nodejs-client (and its submodules) with webpack

We should have a document that clearly explains how to use the library (or one of the submodules from the library) with webpack.

@JustinBeckwith says:

> For future tracking though - my proposal (I think) is that we do the npm run webpack as part of npm run compile, and package the results in the npm module. I'm not 100% sure of if/how webpack supports the "browser" field "spec", but if it does we should consider using that instead.

Refs: https://github.com/googleapis/google-api-nodejs-client/pull/2581