Tidy up how our sass and js is imported

[owenatgov]

What

Makes 3 changes across our sass and js importing and initialisation:

1. Removes the workaround to issues with our treeshaking introduced in Only import the js from GOV.UK Frontend that the website is using #3766 now that Fix tree-shaking when importing from 'govuk-frontend' govuk-frontend#4957 is resolved and published
2. Uses the createAll method to initialise our js following it's introduction in Move responsibilities from initAll to the components govuk-frontend#4945
3. Removes all from the sass import paths as per the changes in Rename any _all.scss files to _index.scss so they work as index files govuk-frontend#4960

Why

This is a redux of #3732 following all the work done via #3731 in the last cycle.

Notes

There are no changes in file size for compiled js and css assets between main and this branch. The primary benefit is that the build now no longer spits out a bunch of sass warnings and our imports are a bit neater.

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[36degrees]

Lovely stuff, thanks for doing this 🙌🏻