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

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

build: bump npm-run-all from 4.1.2 to 4.1.3

build: bump html-minifier from 3.5.8 to 3.5.15

Bumps [npm-run-all](https://github.com/mysticatea/npm-run-all) from 4.1.2 to 4.1.3.
- [Release notes](https://github.com/mysticatea/npm-run-all/releases)
- [Commits](https://github.com/mysticatea/npm-run-all/compare/v4.1.2...v4.1.3)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [html-minifier](https://github.com/kangax/html-minifier) from 3.5.8 to 3.5.15.
- [Release notes](https://github.com/kangax/html-minifier/releases)
- [Commits](https://github.com/kangax/html-minifier/compare/v3.5.8...v3.5.15)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [eslint-plugin-import](https://github.com/benmosher/eslint-plugin-import) from 2.8.0 to 2.11.0.
- [Release notes](https://github.com/benmosher/eslint-plugin-import/releases)
- [Changelog](https://github.com/benmosher/eslint-plugin-import/blob/master/CHANGELOG.md)
- [Commits](https://github.com/benmosher/eslint-plugin-import/compare/v2.8.0...v2.11.0)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Refactor buildSpriteString and update sprite docs