Switch to babel.

[rtibbles]

Summary

• Many moons ago we switched to buble to speed up our webpack builds
• Buble is now in maintenance mode
• Babel seems to have got a lot faster
• This PR makes us use babel again
• Unfortunately Babel is still slow for some things, so I added an exclusion for any files marked as 'dist' to avoid transpiling already transpiled code

Reviewer guidance

• Build completes?
• Manual testing shows no obvious regressions?

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[nucleogenesis]

I'm not a Babel config expert - but everything here LGTM.

Smoke tested it - no issues with the build completing.

However, Babel is far slower (on my machine anyway) - but I get wanting to use maintained software instead of maintenance mode software moving forward here.

Babel:

Kolibri Build: Initial build complete in 87.608 seconds

Buble:

Kolibri Build: Initial build complete in 55.388 seconds

[rtibbles]

Yeah - unfortunately Babel will never be as quick as Buble because Babel does everything. One advantage is that we can start using async/await in future!