Fix release with breaking changes

Some days ago I've dropped the support of Vue 2 for the support of Vue 3, but I've failed my release.
It has been released under a new minor version (3.1.0) instead of a new major version (4.0.0), see #394.

This release 3.1.1 revert these breaking changes and allow Vue 2 again, see https://www.npmjs.com/package/vue-numerals/v/3.1.1.