feat(client): Config nav support custom target

[kev1nzh37]

closes #1595

[watonyweng]

Hi @kev1nzh37, i think the value of target should be limited to [_self, _blank, _parent, _top], please refer to here for more information.

[kev1nzh37]

Hi @kev1nzh37, i think the value of target should be limited to [_self, _blank, _parent, _top], please refer to here for more information.

I have considered this, but will it affect the following situations.

<iframe name="child1" />
<iframe name="child2" />

// vitepress in child1 
<a href="http://www.child1.com/docs1" target="child2">Open</a>

Hope to get more suggestions, thanks.

[brc-dd]

Allow overriding rel too as people might have security concerns.

[brc-dd]

Hey, merged this as #1993. Had to rebase due to conflicts.