feat($theme-default): support configuring target and rel for nav links (close #1353)

[billyyyyy3320]

fix #1353

Summary

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:

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• Yes
• No

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The PR fulfills these requirements:

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• Related documents have been updated
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Other information:

[flozero]

Hello @newsbielt703 thank's for the PR ! I am waiting for others approval and we will merge it

[kefranabg]

Hey @newsbielt703,

If a link is considered as an internal link, adding `target: '_blank' is not working.

For example, in the official VP documentation in .vuepress/nav/en.js :

{
    text: 'Guide',
    link: '/guide/',
    target: '_blank',
  }

This will not make the guide page open in a new tab.

[billyyyyy3320]

@kefranabg Thanks for your suggestion.

[vue-bot]

Hey @newsbielt703, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚