Define new package directories, filenames, extensions

[romaricpascal]

What

Agree on what the published package should look like for v5, focusing on what versions of the component JavaScript (and other entry points) need to exist, including:

• directory structures
• file names and extensions
• what transformations need to be made to each version (transpilation, addition of polyfills)

Draft updated documentation for Importing JavaScript that shows how and when the various versions of the JavaScript would be used by teams.

Why

Build a shared vision for the package so we know where we're trying to get to.

We can use the agreed package structure and draft documentation when communicating the upcoming changes to the community, allowing us to get feedback and reduce the risk around the changes.

Who needs to work on this

Developers, Tech writers

Who needs to review this

Developers, Tech writers

Done when

### We've agreed on

Tasks

Beta Give feedback

1. Folder structures
2. Necessary transformations
3. File extensions

Tasks

Beta Give feedback

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

[colinrotherham]

Decision record for this issue:

• Add decision record about JavaScript file names and formats govuk-design-system-architecture#28

I've updated the tasks with remaining documentation changes

[colinrotherham]

That's all the outstanding changes to govuk-frontend ticked off in #3950

Next it's the Frontend documentation site

[colinrotherham]

Going to close this one 🙌

Realised we've double-task-listed the Frontend Docs changes in:

• Update installation instruction to reflect new JavaScript loading strategy govuk-frontend-docs#308