feat(theme-default): sync anchor permalink function updates for better a11y

…(vuepress#1363)

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[X] Read the Contributing Guidelines.
[X] Provide a description in this PR that addresses what the PR is solving. If this PR is going to solve an existing issue, please reference the issue (e.g. close #123).

What is the purpose of this pull request?

[X] Bug fix
[ ] New feature
[ ] Other

Description

The heading anchors have aria-hidden attributes but are still focusable via keyboard navigation. There was a bit of discussion around this already in the markdown-it-anchor repo and you can address this by just using the headerLink render function instead of the ariaHidden render function since it wraps the entire header in an anchor.

This PR is the theme-default style updates for the markup changes from PR #1452 in vuepress-next.

This fixes #1363