fix: reduce bundle size

[merceyz]

• Changed the build process to make babel helpers external to reduce code duplication in projects using vuex-map-fields.
• Made rollup handle all the different outputs in one config file
• Removed uglify-es and used rollup-plugin-terser instead
• Removed redundant config in babel.config.js, rollup-plugin-babel already handles modules: false

[merceyz]

@maoberlehner Should probably set corejs on @babel/preset-env so it includes polyfills

[coveralls]

Coverage remained the same at 100.0% when pulling 93abd42 on merceyz:bundle-size into fa5c480 on maoberlehner:master.