fix(rollup): automatically find external dependencies

You are currently hardcoding all external module names in a list in the Rollup build. This is a tedious and error-prone approach, as you have to manually make sure the list is always up to date with the packages that are actually available.

This PR automates the list by automatically getting the names of all internal node modules, all npm dependencies, and all npm peer dependencies. Making this change also allowed me to catch a current bug where you rely on jest-diff in the final package, but haven't defined that package as a dependency anywhere.

This change is part of the fix for #124 (closed)