feat(esm): use native ESM in StrykerJS

[nicojs]

Use native ESM in StrykerJS itself.

TODO:

• Migrate packages
  • grunt-stryker
  • api
  • test-helpers
  • util
  • instrumenter
  • typescript-checker
  • mocha-runner
  • jasmine-runner
  • jest-runner
  • karma-runner
  • cucumber-runner
  • core
• Re-enable code coverage: nyc test coverage not working when type:module is activated istanbuljs/nyc#1353 (comment)
• Make sure mutation testing runs as expected (change to stryker.conf.cjs).
• Update dependencies that migrated to pure ESM to their latest version. Based on dependabot PR's: chalk, execa, get-port, decamelize, npm-run-path (are we using this?), commander
• Enable "module": "ES2020" from root tsconfig (instead of in each child tsconfig)
• Enable linting & formatting on all verify/verify,js files
• Update docs (require -> import)

Find-replace helper regex:

• Find: from '\.\.\/(.+)'; replace: from '../$1.js';
• Find: from '\.\/(.+)'; replace: from './$1.js';

BREAKING CHANGE: StrykerJS is now a pure ESM. Please read this.
BREAKING CHANGE: Node 12.20 is now the min version.
BREAKING CHANGE: Karma v6.3 is now the min supported karma version for @stryker-mutator/karma-runner, because we need async config: https://github.com/karma-runner/karma/blob/master/CHANGELOG.md#630-2021-03-23

Closes #3230
Closes #3397