[WIP] feat: markdown.extractHeaders option

[ulivz]

Summary

Fix #1903

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:

module.exports = {
  markdown: {
    extractHeaders: ['h2', 'h3', 'h4'] // defaults to ['h2', 'h3'] 
  }
}

Replacement of #1111

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:

This is my 1st work with Gitpod

[funnycoderstar]

This is exactly what I want.

[ulivz]

Issue of this pull request is that sidebar of VuePress doesn't support infinite nesting. even we extract more headers, extra headers will not be indented, which would be considered as a new issue.

Maybe we need to fix the indent problem before getting this pull request merged.

// cc @kefranabg @f3ltron

[kefranabg]

Seems like I do not have rights to push on this PR 🤔 @funnycoderstar could you enable authors commits on your fork?

[kefranabg]

Close this PR in favor of #1945 as I'm not able to update this one.