refactor(client): load more lazily

[caugner]

Summary

Problem

Our main bundle is larger than it could be.

Solution

Load more pages lazily.

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Screenshots

Before

  156.44 kB  build/static/js/main.1b11e75b.js
  28.42 kB   build/static/css/main.8782317e.css

After

  128.77 kB  build/static/js/main.cfa13c20.js
  21.99 kB   build/static/css/main.a25ea272.css

---

How did you test this change?

Ran yarn build:client.

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.

[caugner]

Lazy-loading impacts SSR, so closing for now, see also: https://github.com/mdn/yari/pull/11048