Update README and CHANGELOG for ES module bundle

[colinrotherham]

This PR changes our documentation to use the ES module govuk-frontend.min.js bundle:

1. Update the README "Importing JavaScript" documentation
2. Update CHANGELOG with breaking change to swap all.js with ES modules

The previous default all.js became all.bundle.js in f8335d9 as part of:

• Define new package directories, filenames, extensions #3482

[claireashworth]

Suggested change

	initialising JavaScript in your application to ensure that all users can use it successfully.
	initialising JavaScript in your application. This will ensure all users can use it successfully.

[claireashworth]

Just 1 minor change suggestion

[colinrotherham]

Thanks @claireashworth, all sorted

[colinrotherham]

Mind if I request a review from @36degrees @romaricpascal @domoscargin?

Since we link to the advanced options I've assumed the default ES module bundle was enough

[More details on importing JavaScript and advanced options](https://frontend.design-system.service.gov.uk/importing-css-assets-and-javascript/#javascript)

But do we need to specifically need to add:

1. Action for window.GOVUKFrontend users to now use UMD all.js all.bundle.js
2. Action for Rails Asset Pipeline users to change UMD *.js to *.bundle.js

[domoscargin]

Mind if I request a review from @36degrees @romaricpascal @domoscargin?

Since we link to the advanced options I've assumed the default ES module bundle was enough

[More details on importing JavaScript and advanced options](https://frontend.design-system.service.gov.uk/importing-css-assets-and-javascript/#javascript)

But do we need to specifically need to add:

1. Action for window.GOVUKFrontend users to now use UMD all.js all.bundle.js
2. Action for Rails Asset Pipeline users to change UMD *.js to *.bundle.js

I think it's OK to just include instructions for the default way we want folks to to use, but maybe we could highlight the advanced options more, like:

If you currently import the JavaScript in a different way, you may need to make some changes. See more details on importing JavaScript.

(but with better words 🤣 )

[colinrotherham]

Ha, thanks. Any thoughts @claireashworth?

[claireashworth]

Yup, highlighting where they can find more detailed info is better than too much information front loading. A slight tweak to wording could be:

If you import JavaScript using a different method, you might need to make some changes. Refer to the detailed guidance on importing JavaScript.

[colinrotherham]

Thanks @claireashworth. Mind if I widen that link text to include "detailed guidance on"?

If you import JavaScript using a different method, you might need to make some changes. Refer to the detailed guidance on importing JavaScript.

Pushed it up

[domoscargin]

LGTM!