feat(esm config): support config file as pure esm

[nicojs]

Support your stryker.conf.js file to be a pure esm when { "type": "module" } in your package.json file.

Also add support for 2 new default configuration file names: stryker.conf.mjs and stryker.conf.cjs.

• Implement new algorithm in the config reader
• Add unit tests and update integration tests
• Update the docs to include export default { }
• Update stryker init to include auto-detect package types and choose the correct format.

BREAKING CHANGE: module.exports = function(config) {} is no longer supported in your stryker.conf.js file. This was already deprecated, but now will give an error.

Closes #3408

[nicojs]

@simondel I've decided that stryker init will output a stryker.conf.mjs file from Stryker 6 onward. That way we don't need to support 2 js output formats in the stryker init command.

Loading stryker.conf.mjs files is supported from nodejs 12 onward, so it should be no problem for Stryker@6