Enable official support for React 18

[MMMalik]

This PR officially enables support for React 18. The following work has been done:

• React 18 is now supported via peerDependencies field in package.json.
• Unit tests are adjusted according to new API of [React Testing Library].(https://testing-library.com/docs/react-testing-library/intro/), especially with regards to React Hooks
• Slight updates to karma config.
• Dependencies are upgraded, if possible (one exception is e.g. chalk which relies on ESM since v5).
• TypeScript version range is now limited to patch updates (TS doesn't follow semantic versioning).

Closes #284.

[MMMalik]

This is ready for review.

One more considration to be taken into account: React 18 drops support for IE11 :see_no_evil:. So if we enable support for React 18 exclusively, then we also basically drop support for IE11 in React integration.

---

There are still alternative routes we can take. For instance, we can enable support for React from 16.8.x to 18.x AND loosen our testing policies:

• Run full suite of unit tests for React 18.x and E2E tests for selected versions from 16.8.x to 18.x
• OR: Run full suite of unit tests for selected versions from 16.8.x to 17.x and E2E tests for selected versions from 16.8.x to 18.x

This way we could cover with tests all versions our source code would work on and unlock official support for React 16.8 to 18.x via peerDependencies:

"peerDependencies": {
    "ckeditor4": "^4.19.0",
    "react": "^16.8 || ^17 || ^18"
},

[MMMalik]

This is ready for another round of review.

Few changes were introduced:

• Problematic ESLint rules are disabled since one of them might introduce breaking changes downstream (import type ...).
• Deps are updated in package.json (root and samples) to newest versions.
• README is updated to be in sync with docs update.

[MMMalik]

• As discussed on Slack, old link to documentation is brought back.
• Samples are adjusted to use new react-dom/client API.
• Polyfills for IE11 are removed from samples.