Differential serving using module/nomodule scripts

[jroebu14]

Resolves #7155

Overall change:
Simorgh now builds 2 client side JavaScript bundles. A slimmer modern bundle for modern browsers and a (currently live) bloated bundle for legacy browsers.

The modern bundle is transpiled to ES2017 syntax and legacy bundle is transpiled to ES5.

More info in the docs

Code changes:

• Updates webpack and babel config to handle building 2 different client-side bundles - legacy and modern
• Updates the server side Document component to render both legacy and modern bundles and conditionally load them using module/nomodule.

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• I have assigned myself to this PR and the corresponding issues
• I have added the cross-team label to this PR if it requires visibility across World Service teams
• I have assigned this PR to the Simorgh project
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Testing:

Requires full cross-browser regression testing.

To test please run yarn build && yarn start

• Automated (jest and/or cypress) tests added (for new features) or updated (for existing features)
• If necessary, I have run the local E2E non-smoke tests relevant to my changes (CYPRESS_APP_ENV=local CYPRESS_SMOKE=false yarn test:e2e:interactive)
• This PR requires manual testing

[amoore108]

This looks great 👍🏼 It'll be interesting to see the effect this will have on our performance metrics. Is it worth capturing or referencing benchmarks before these changes are implemented and then capturing some afterwards?

[DarioR01]

LGTM

[jroebu14]

@amoore108 we have a couple of tools for monitoring performance. We have both real user measurements and synthetic tests running daily. I was planning on looking at this data. Is there any other method of measuring performance you think would be useful?

[amoore108]

@amoore108 we have a couple of tools for monitoring performance. We have both real user measurements and synthetic tests running daily. I was planning on looking at this data. Is there any other method of measuring performance you think would be useful?

That sounds good to me! Just wanted to check if we were capturing any comparison data before merge 😄