refactor: externalize markdown headers plugin

[meteorlxy]

No description provided.

[brc-dd]

Regarding this header plugin, do we actually need it? Because we're just using it for the aside outline, but using headers extracted from markdown is preventing users from using dynamic headers (having Vue-style interpolations / added using some component). In future, we'll probably need to make that component client-only so that it can handle such headings.

There shouldn't be any need at our side of this, but I am not sure if people are using this stuff. It was never publicly exposed though. In (useData) docs too, VitePressData has no headers property.

EDIT:

It looks like the vuejs/theme repo is using this stuff, so lets keep it. In the default theme, we can just update the component, keeping data stuff same.