karmatic

With the new E2E test suite, I noticed that coverage is not properly reported when the user provides a custom webpack.config.js. This PR fixes that by detecting if webpack loaders exist and if so, adding a new babel-loader with just the coverage plugin enabled. Our tests also now enforce that coverage is correctly reported for all tests.

Couple of decisions I made up I wanted to call out: 1. I decided to include and plugins as deps so our default rollup config is mildly useful. Was thinking for uses with where there isn't a for us to use. Would be curious to get other's thoughts here on what plugins we should include. 2. If a rollup config is present, then I just use that unmodified. I'm not sure if rollup provides an API to modify the config/options of plugin instances from a config object. I think this method (of just using the provided config unmodified) is fine though. I think the only thing lost right now is code coverage (though I guess I could just add another babel plugin instance with only the coverage plugin configured...) This is still a work in progress but thought I'd put an early draft out here to get some feedback. I also need to update the testing infra a bit so I can validate this implementation but that's gonna come in a different PR.