feat: add permalink variable timestamp

[uiolee]

What does it do?

fix #5586

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.
• docs(Permalinks): add :timestamp site#2326

[github-actions]

How to test

git clone -b timestamp https://github.com/hexojs/hexo.git
cd hexo
npm install
npm test

[coveralls]

Pull Request Test Coverage Report for Build 12764580920

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.482%

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Totals
Change from base Build 12702421469:	0.0%
Covered Lines:	9411
Relevant Lines:	9460

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💛 - Coveralls

[SukkaW]

Should we add :update_timestamp as well IMHO?

[uiolee]

Should we add :update_timestamp as well IMHO?

I'm not sure if there is such a use case. But generally speaking, permalink should not change often

[SukkaW]

Just a thought, we could use an approach like the following to only calculate the permalink value when needed, and cache it afterward:

function defineLazyProperty(object, propertyName, valueGetter) {
  const define = value => Object.defineProperty(object, propertyName, {value, enumerable: true, writable: true});
  Object.defineProperty(object, propertyName, {
    configurable: true, enumerable: true,
    get() {
      const result = valueGetter();
      define(result);
      return result;
    },
    set: define
  });
  return object;
}

const permalink = {};

defineLazyProperty(permalink, 'second', () => date.format('ss'));
// ...
defineLazyProperty(permalink, 'timestamp', () => date.format('X'));

Or we can use Map instead of object here.

We can do this in another PR.