Add example of how to use an npm commonjs package like date-fns in ts_devserver & ts_web_test_suite - #343

This works for shallow imports from npm packages such as import {format} from 'date-fns' but it doesn't handle deep imports such as import * as format from 'date-fns/format'. For that you'd need to browserify a separate UMD package that defines the date-fns/format module. It can be done but would result in some duplicate code being loaded if you're using both shallow and deep imports of an npm package.

Could you add some text here so ppl know this is a temp solution and we plan something more ergonomic for 8?