feat(theme): allow joining attr to external link

[WizardsBowl]

Description

I tried to add rel="nofollow" attribute to an external link and found it doesn't work.

My markdown file is like this:

[LINK](https://external.link){rel=nofollow} // use markdown-it-attrs plugin

And this is the final html:

<a href="https://external.link" rel="noreferrer" target="_blank">LINK</a>

It's rel attribute is an unexpected noreferrer but not nofollow.

That's because the linkPlugin always use attrSet and override the old value.

This PR add an option telling the linkPlugin to use attrSet or to use attrJoin for each attr.

linkPlugin(
  md,
  {
    target: '_blank',
    rel: { value: 'noreferrer', join: true },
    example1: 'will-override',
    example2: { value: 'will-override' },
    example3: { value: 'will-override', join: false },
    example4: { value: 'will-join-in', join: true },
    ...options.externalLinks
  },
  base,
  slugify
)

let externalLinks?: Record<string, string | ExternalLinkAttrValue>

export interface ExternalLinkAttrValue {
  value: string
  join?: boolean
}

The final html contains both nofollow and noreferrer:

<a href="https://external.link" rel="nofollow noreferrer" target="_blank">LINK</a>

Linked Issues

None.

Additional Context

The markdown-it-attrs plugin is built-in with VitePress.

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