refactor(theme-classic): move some logic of CodeBlock to theme-common

[Josh-Cena]

Motivation

To make the diff in #5783 less confusing. Moved all highlight-related code to theme-common.

Not sure if we should be less opinionated and pass the directives as params instead of hard-coded? Is there a customization need here?

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Preview; rendered result still the same.

[netlify]

✔️ [V2]

🔨 Explore the source changes: 3e7b146

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/618d1e15dcaeb400083bdddd

😎 Browse the preview: https://deploy-preview-5922--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	92
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5922--docusaurus-2.netlify.app/

[github-actions]

Size Change: +2.14 kB (0%)

Total Size: 884 kB

Filename	Size	Change
website/build/assets/js/main.********.js	476 kB	+1.85 kB (0%)
website/build/blog/2017/12/14/introducing-docusaurus/index.html	66.1 kB	+42 B (0%)
website/build/docs/installation/index.html	51.7 kB	+220 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	38.3 kB	0 B
website/build/assets/css/styles.********.css	94.2 kB	0 B
website/build/blog/index.html	36.8 kB	0 B
website/build/docs/index.html	44 kB	+29 B (0%)
website/build/index.html	29.5 kB	0 B
website/build/tests/docs/index.html	25.2 kB	0 B
website/build/tests/docs/standalone/index.html	21.7 kB	0 B

_{compressed-size-action}

[slorber]

Thanks 👍

Not sure if we should be less opinionated and pass the directives as params instead of hard-coded? Is there a customization need here?

We'll figure out if anyone complains and present a usecase ;)