A version NPM script has been added that runs a new build/package.js
script, which generates a package.json5 file with the new version
number. The package.json5 file is added to the index before committing.

Added a new NPM script named build-package for generating the
package.json5 file.

Node.js v6 supports ES5 features, which removes the need for Babel. The
following changes and improvements have been made.

- Development has been moved from the src folder to the lib folder. The
  src folder has been removed.
- ES Module imports/exports have been replaced with CommonJS equivalents.
- ESLint now checks for unsupported Node.js features.
- All Babel dependencies have been removed.
- Generating lib/unicode.js is no longer done on every build. It is now
  done using `npm run build-unicode`. It is only necessary when the
  version of Unicode has changed.
- NPM scripts have been simplified.

This commit also:

- Updates dependencies
- Simplifies NPM scripts

fix typo in Readme (Contibuting)

Use rollup-plugin-terser instead of rollup-plugin-uglify since uglify-es
is no longer supported and uglify-js doesn't support ES6+.