Webpack trainees.js with a separate config file.

[biancadanforth]

PR #45 adds a new trainees.js script used for training Fathom. The current training process (as described in comments in trainees.js) involves manually copying over three files into the fathom-trainees add-on.

This is a little tedious, so it'd be nice if trainees.js and all its dependencies could be bundled into a single trainees.js file to be used for training. This training Webpack config would be different from our WE Webpack config (webpack.config.js) in a few notable ways:

• Entry: trainees.js
• Rules: Run only the ./extraction/ruleset_factory.js file through the babel-loader to enable use of the autobind decorator.
• Output: Also call the bundled file trainees.js (rather than [name].bundle.js) to work with the fathom-trainees add-on and save the bundled file potentially to a training subfolder in ./src.
• Configure Webpack such that it doesn't transform imports from specific libraries.
  • We wouldn't want Webpack to bundle fathom-web into the training bundle since those imports get processed by rollup in the fathom-trainees add-on.
• This would also enable the use of the autobind decorator for the RulesetFactory class exported by ruleset_factory.js.