[Feature @vuepress/plugin-blog] pageEnhancers add no exists page

[lhz960904]

Summary
reason：when I create a blog theme, I need archive Page, my config is

module.exports = {
  // ...
  plugins: [
    ['@vuepress/blog', {
      pageEnhancers: [
        {
          when: ({ regularPath }) => regularPath === '/archive/',
          frontmatter: { layout: 'Archive' },
        }
      ]
    }]
  ]
}

but archive page is not success, because pulgin-blog just executing ctx.pagesforEach, so, my idea is add permalink parameter and filter noExistsPage to addPage。

// ... 
const noExistsPage = pageEnhancers.filter(it => it.permalink).map(it => ({ permalink: it.permalink }))
const extraPages = [
// ...
...noExistsPage
]

May be it is not good method, It's just a proposal. thanks

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[ulivz]

Closing this PR in favor of #1180

[lhz960904]

Closing this PR in favor of #1180
😃ok