fix: preserve line breaks when copy code in Firefox

[lex111]

Motivation

Should fix #5663

Let's use cross-browser old way using <br> tags to show line breaks in code blocks instead of relying on CSS.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Try copy code manually in different browser.

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[netlify]

✔️ [V2]

🔨 Explore the source changes: 757632b

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/61606235b9300400087d74e2

😎 Browse the preview: https://deploy-preview-5668--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	97
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5668--docusaurus-2.netlify.app/

[github-actions]

Size Change: +339 B (0%)

Total Size: 837 kB

Filename	Size	Change
website/build/assets/css/styles.********.css	93.9 kB	-35 B (0%)
website/build/blog/2017/12/14/introducing-docusaurus/index.html	67.2 kB	+188 B (0%)
website/build/docs/installation/index.html	53.6 kB	+177 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	38.3 kB	0 B
website/build/assets/js/main.********.js	421 kB	0 B
website/build/blog/index.html	38.1 kB	0 B
website/build/docs/index.html	45 kB	+9 B (0%)
website/build/index.html	30.8 kB	0 B
website/build/tests/docs/index.html	26.2 kB	0 B
website/build/tests/docs/standalone/index.html	22.9 kB	0 B

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[slorber]

LGTM 👍 seems to work fine thanks