[Breaking] Babel 7 + infrastructure updates.

[ryan-roemer]

This supersedes the much appreciated existing work on babel updates to hopefully get us into a good, modern state.

• BREAKING: Change plugin options to only be a real JS object. Removes string configuration path option as now this is all possible with dynamic .babelrc.js or babel.config.js files.
• BREAKING: Update to @babel/core / Babel 7+. Fixes #43 Closes #49, #50
• Lint all test code. Most diffs in this PR are those changes. Fixes #56
• Update other dependencies. Closes #55
• Remove dependency on babel6-only test library https://github.com/walmartlabs/assert-transform and do a simple function replacement.

Would ❤️ review from the folks whom have opened PRs / issues / comments to see if this now meets our needs and use cases (particularly the dynamic config options) -- @xcarpentier @dcalhoun etc.

Thanks!!! 🙏

[ryan-roemer]

Released in babel-plugin-transform-define@2.0.0