feat: meta_generator helper

[SukkaW]

What does it do?

This is a continuation of #3669

How to test

git clone -b meta_generator https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

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

[coveralls]

Coverage increased (+0.002%) to 97.269% when pulling c5e4a45 on SukkaW:meta_generator-helper into 52adf39 on hexojs:master.

[curbengh]

What's the benefit of having this (to the theme) instead of meta_generator: true|false?

[SukkaW]

@curbengh
If a theme want to add meta_generator tag, this helper will be helpful. With theme using this helper, the meta_generator filter can be disabled and speed up generation.

[curbengh]

Fair enough.

Then,

hexo/lib/plugins/filter/after_render/meta_generator.js

Line 8 in 3a3e101

const hexoGeneratorTag = `<meta name="generator" content="Hexo ${this.version}">`;

can be refactored to const hexoGeneratorTag = metaGeneratorTag(); to de-duplicate.

[SukkaW]

@curbengh Since it is only a simple string, I see no necessary replacing it with a function.