build: bump core-js from 2.5.3 to 2.5.6

Prevents webpack from referencing `window` in the UMD build which was causing an error in node.

Bumps [core-js](https://github.com/zloirock/core-js) from 2.5.3 to 2.5.6.
- [Release notes](https://github.com/zloirock/core-js/releases)
- [Changelog](https://github.com/zloirock/core-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/zloirock/core-js/compare/v2.5.3...v2.5.6)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Removes polyfills for `Object.assign` and `Set` which, as far as I can tell, are unused in the
codebase. Updates `webpack` from 3 to 4. Replaces `babel-preset-es2015` with `babel-preset-env`.
Reduces the minified bundle size (`feather.min.js`) by **9.3kB** (72.4kB to 63.1kB). Does not change
any functionality of the library.

build: bump eslint-plugin-import from 2.8.0 to 2.11.0