Support folding code blocks in tabs

[PeichengLiu]

PR Checklist

• The commit message follows guidelines for NexT.
• The changes have been tested (for bug fixes / features).
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Improvement.
• Code style update (formatting, linting).
• Refactoring (no functional changes).
• Documentation.
• Translation.
• Other... Please describe:

What is the current behavior?

Non-default tab is not folded.
See also #679.

What is the new behavior?

Non-default tab is folded on the fly based on the event dispatched when user switch to a new tab.

• Link to demo site with this changes: https://peichengliu.github.io/kuangbin/kuangbin-%E4%B8%93%E9%A2%98%E5%9B%9B-%E6%9C%80%E7%9F%AD%E8%B7%AF%E7%BB%83%E4%B9%A0/
• Screenshots with this changes:

How to use?

In NexT _config.yml:

codeblock:
  # Fold code block
  fold:
    enable: true
    threshold: 500

[welcome]

Thanks so much for opening your first PR here!

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Pull Request Test Coverage Report for Build 5954202769

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.416%

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Totals
Change from base Build 5728719282:	0.0%
Covered Lines:	394
Relevant Lines:	399

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💛 - Coveralls

[stevenjoezhang]

Thanks for your contribution!

[welcome]

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