Configure ES modules properly

[Robdel12]

It is common in a webpack environment when using babel to not transform modules within the node_modules directory. Our package's "module" field points directly to the src files, which are typically compiled with @babel/preset-env. When babel is configured to not compile our packages within node_modules, bundles that use our packages can end up broken.

This can be seen in @bigtest/interaction tests by removing the (!?/@bigtest) regexp within the babel-loader exclude option in the karma.config.js file. This causes the package to fail to properly extend the convergence class and thereby causes every test to fail.

(moved from thefrontside/bigtest#55)