[Feature request] Provide isExternal options for `markdown.links`

[Mister-Hope]

Clear and concise description of the problem

In some cases, people may want to deploy multiple VuePress sites (or sites generated by other tools) in one directly.

For example:

• /: a vuepress site
• /api/: a site generated by typedoc

In this case, any links that is under /api/ should be marked external, however, the linksPlugin uses isLinkExternal from @vuepress/shared directly here:

core/packages/markdown/src/plugins/linksPlugin/linksPlugin.ts

Line 76 in eaa97e6

if (isLinkExternal(hrefLink, base)) {

Suggested solution

Provide a isExternal option:

isExternal: (link: string, env: MarkdownEnv) => boolean

We can also use other formats, but it must expose base filePathRelative variables so that users can "resolve" final path to make a further judgement if necessary.

Alternative

No response

Additional context

No response