perf: reduce the number of match calls in metaGenerator

[D-Sketon]

What does it do?

If we have already determined during the initial check that this.config.meta_generator is true and the user has not inserted an additional generator meta tag, then we do not need to repeat this check on every subsequent page.

Screenshots

Pull request tasks

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

[github-actions]

How to test

git clone -b perf/8.0.0-10 https://github.com/D-Sketon/hexo.git
cd hexo
npm install
npm test

[coveralls]

Pull Request Test Coverage Report for Build 18442055605

Details

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.526%

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Totals
Change from base Build 18259215002:	0.0%
Covered Lines:	9868
Relevant Lines:	9915

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💛 - Coveralls

[SukkaW]

Is it even possible to self-unregister the filter? No if is always faster than having if. If we determined the filter is not needed, we can simply skip/unregister the register.