fix($theme-default): Expand nested sidebar groups

[timaschew]

Currently only direct children of a groups are checked (if any of them is active).
This fix checks the children recursively.

Fix #1497

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:

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

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:

[flozero]

Thanks for your work @timaschew. Can you add some test to validate what you mean please

[timaschew]

@f3ltron are you talking about to write a test case (code) or about a demo to show?

Later can be viewed here:
Problem: https://vuepress-fix-demo.herokuapp.com/sidebar-problem/docs/subsec/subsubsec/
Fix: https://vuepress-fix-demo.herokuapp.com/sidebar-fix/docs/subsec/subsubsec/

[flozero]

Oh ok i definitely see the improvment now ^^. Just let me some time to review it and i will speak with the others about it. You can start by adding / updating the doc part about it

[timaschew]

You can start by adding / updating the doc part about it

What do you mean? This is a bug for me so I wouldn't assume it require to change any documentation.

[flozero]

I will have a look about it. It's look it was actually not supported by Vuepress. Thank's for the PR

[flozero]

https://github.com/timaschew/vuepress-nested-sidebar-demo/blob/master/.vuepress/config.yml

[kefranabg]

@timaschew Thanks for this PR 💪 Could you just accept my code suggestions to improve code readability

[timaschew]

Hi @kefranabg thanks for the code review! Sure I've confirmed your suggestions.
Let me know if you expect a rebase/squash by me.

[kefranabg]

Awesome. Thanks @timaschew

[vue-bot]

Hey @timaschew, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚