Add preload tag for main.css

[njzjz]

Improve performance. In addition, CloudFlare has supported early hints with this tag.
See https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types/preload
See also https://blog.cloudflare.com/early-hints-on-cloudflare-pages/

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

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

What is the current behavior?

Issue resolved:

What is the new behavior?

• Link to demo site with this changes:
• Screenshots with this changes:

How to use?

preconnect: true

[coveralls]

Pull Request Test Coverage Report for Build 3318360491

• 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 97.614%

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Totals
Change from base Build 3174130733:	0.0%
Covered Lines:	394
Relevant Lines:	399

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💛 - Coveralls

[stevenjoezhang]

Does this feature need to be associated with the preconnect option in _config.yml?

hexo-theme-next/_config.yml

Lines 339 to 341 in 1e0f28d

	# Preconnect CDN for fonts and plugins.
	# For more information: https://www.w3.org/TR/resource-hints/#preconnect
	preconnect: false

[njzjz]

Does this feature need to be associated with the preconnect option in _config.yml?

Done. For compatibility, I don't change the option name.