Add a new polyfill

• The polyfill implementation should be added to the packages/core-js/modules directory.
• Shared helpers should be added to the packages/core-js/internals directory. Reuse already existing helpers.
• For export the polyfill, in all common cases use internals/export helper. Use something else only if this helper is not applicable - for example, if you want to polyfill accessors.
• If the code of the pure version implementation should significantly differ from the global version (that's not a frequent situation, in most cases internals/is-pure is enough), you can add it to packages/core-js-pure/override directory. The rest parts of core-js-pure will be copied from core-js package.
• Add the feature detection of the polyfill to tests/compat/tests.js, add the compatibility data to packages/core-js-compat/src/data.mjs, how to do it see here, and the name of the polyfill module to packages/core-js-compat/src/modules-by-versions.mjs (this data is also used for getting the default list of polyfills at bundling and generation indexes).
• Add it to entry points of those directories where it's required:
  • packages/core-js/es
  • packages/core-js/stable
  • packages/core-js/actual
  • packages/core-js/full
  • packages/core-js/proposals
  • packages/core-js/stage
  • packages/core-js/web.
• Add unit tests to tests/unit-global and tests/unit-pure.
• Add tests of entry points to tests/entries/unit.mjs.
• Make sure that you are following our coding style.
• Document it in README.md, CHANGELOG.md and docs

Style and standards

The coding style should follow our ESlint configuration. You can test it by calling npm run lint. Different places have different syntax and standard library limitations:

• Polyfill implementations should use only ES3 syntax and standard library, they should not use other polyfills from the global scope.
• Unit tests should use the modern syntax with our minimalistic Babel config. Unit tests for the pure version should not use any modern standard library features.
• Tools, scripts and tests, performed in NodeJS, should use only the syntax and the standard library available in NodeJS 8.

File names should be in the kebab-case. Name of polyfill modules should follow the naming convention namespace.subnamespace-where-required.feature-name, for example, esnext.set.intersection. The top-level namespace should be es for stable ECMAScript features, esnext for ECMAScript proposals and web for other web standards.