refactor(core): replace useDocusaurusContext().isClient by useIsBrowser()

[slorber]

Breaking change:

• Replace removed useDocusaurusContext().isClient by new useIsBrowser()

Motivation

Perf reasons: isClient is toggled after React hydration, leading to a duplicate re-rendering of all components using useDocusaurusContext(), even if they don't even use the isClient value.
By extracting isClient in a separate context, only components reading that value will render twice on hydration, and the rest will just render once => less work for the browser and better perfs.
More explanations on the problem space here: https://www.joshwcomeau.com/react/the-perils-of-rehydration/

Naming reasons: we have <BrowserOnly> (and not <ClientOnly>) so we'd rather keep a consistent naming for things that happen after React hydration

Also, remove some unexpected usage of isClient as key: we'll see if it produce unintended side-effects.

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

tests + preview

[netlify]

✔️ [V2]

🔨 Explore the source changes: 630f019

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/6115471f1c33c30008353efa

😎 Browse the preview: https://deploy-preview-5349--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	93
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5349--docusaurus-2.netlify.app/

[github-actions]

Size Change: +403 B (0%)

Total Size: 794 kB

Filename	Size	Change
website/build/assets/js/main.********.js	404 kB	+401 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	36.2 kB	0 B
website/build/assets/css/styles.********.css	93.1 kB	+2 B (0%)
website/build/blog/2017/12/14/introducing-docusaurus/index.html	64.2 kB	0 B
website/build/blog/index.html	33.1 kB	0 B
website/build/docs/index.html	41.9 kB	0 B
website/build/docs/installation/index.html	49.6 kB	0 B
website/build/index.html	28.7 kB	0 B
website/build/tests/docs/index.html	22.5 kB	0 B
website/build/tests/docs/standalone/index.html	20.5 kB	0 B

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[lex111]

That's awesome!

[slorber]

thanks 😄

Comparing before/after on lighthouse for a very large page seems to have a significant impact on blocking time / TTI:

Before: https://deploy-preview-5345--docusaurus-2.netlify.app/community/changelog/

After: https://deploy-preview-5349--docusaurus-2.netlify.app/community/changelog/

Going to merge this as I didn't see any weird key related side effect, but let's keep our eyes open 😄