Updates

[brettz9]

• npm: Update estraverse (minor) and devDeps; switch from non-maintained rollup-plugin-babel to @rollup/plugin-babel

The build file needs updating, so I figured we might also ensure the build environment is up to date.

I've made explicit the default babelHelpers: 'bundled' behavior of @rollup/plugin-babel. (Otherwise, with the runtime option, one needs to add a dependency, and without any option, we get a console warning.)

Let me know if you don't want the estraverse dependency bump.