Create release notes for v4.2.0

[domoscargin]

What

Write the release notes for GOV.UK Frontend v4.2.0

Why

To help users understand what changes have been made in the 4.2.0 release.

Who needs to work on this

Tech Writer; Developers

Who needs to review this

Developers; Tech Writer + 2i

Done when

This will be done when the release notes:

• have been drafted
• Made sure to thank any contributors for this release - we should thank the people who raised the ESM issues and help us test pre-releases:
  • 4.1.0 Breaks the ability to import scss files with webpack sass-loader #2645
  • 4.1.0 breaks require.resolve to find all.js client javascript #2643
  • ES Modules can't be imported without a bundler #2633
• have been reviewed by a developer
• have been 2i'ed

[vanitabarrett]

Release notes draft doc here: https://docs.google.com/document/d/1rqBb9vxm7pqycdlCbNwXwKSbfC5SAIjAmeVRhwI-SUQ/edit#