Update polyfilling with core-js

[GertSallaerts]

Problem

• core-js@2 is obsolete and pollutes the global namespace
• @babel/preset-env's corejs: 3 option includes core-js@3 and its polyfills pollute the global namespace too
• @babel/plugin-transform-runtime's corejs: 3 option includes core-js-pure@3 which does not pollute the global namespace but polyfills everything regardless of the targets specified by preset-env.

Would also resolve #342

Solution

By using the new Babel Polyfills we can include modern and pure polyfills based on the targets config (none is specified in twitter-text so it will use the default) in the root of babel config.

[CLAassistant]

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