feat($default-theme): allow 'auto' in defined sidebar (fix #1252)

[bobheadxi]

Summary

This PR allows you to define 'auto' in defined sidebars, to allow you do define groups of pages that should have sidebars automatically generated. This fixes #1252, which has some detailed use cases + rationale from others.

The change only adds an additional case to check specifically for an 'auto' configuration so it shouldn't be breaking

For example:

// .vuepress/config.js
module.exports = {
  themeConfig: {
    sidebar: {
      '/handbook/project-management/': [
        'scope',
        'sprints',
        'repositories',
      ],
      '/handbook/tools/': [
        'slack',
        'github',
        'deployment',
      ],
      '/': 'auto',
    },
}

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 (it does not seem like there is are programmatic tests for the code changed)

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:

I'm not sure how to test this - any guidance would be much appreciated! What I did do to make sure this works:

• copy the contents of util/index.js into my copy of vuepress in my project's node_modules
• verified behaviour with my project using configuration similar to the one above

[bobheadxi]

@ulivz could you please take a look at this? :) it's a very small fix for what I feel like is significant omission from how the sidebar groups currently work, and the original issue (#1252) indicates I'm not the only one

thank you!

[ulivz]

LGTM, Thanks.