refactor(model): add Code model for include_code tag

[D-Sketon]

What does it do?

try to fix #5479 and #5486

ref #5479 (comment)

Since the current model doesn't satisfy include_code, I experimentally added the Code Model

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[github-actions]

How to test

git clone -b fix/code https://github.com/D-Sketon/hexo.git
cd hexo
npm install
npm test

[D-Sketon]

I'm not sure the files in code_dir need to be written to the public folder

[coveralls]

Pull Request Test Coverage Report for Build 13342777776

Details

• 100 of 100 (100.0%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 99.467%

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Totals
Change from base Build 13299585829:	0.005%
Covered Lines:	9892
Relevant Lines:	9945

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💛 - Coveralls