feat: allow VPContent to use custom components

[shigma]

Description

Currently, VPContent hardcodes the use of VPPage, VPHome, and VPDoc for the built-in page, home, and doc layouts. Custom layouts only work via <component :is> for non-built-in layout names. This means there is no way to fully override the built-in layout components from a custom theme.

This PR uses resolveComponent to check whether the user has registered a global component matching the layout name (page, home, or doc). If a matching component is found, VPContent falls through to <component :is> instead of using the built-in component. If no override is registered, the behavior is exactly the same as before.

Linked Issues

N/A

Additional Context

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[pkg-pr-new]

npm i https://pkg.pr.new/vitepress@5176

commit: 4ee0955

[brc-dd]

could've just returned true/false I guess for this use case

[edison1105]

@brc-dd @shigma
It is not appropriate to use resolveComponent to check whether a component exists, as it will trigger a warning in DEV if the component does not exist. If this check is required, it is recommended to use resolveDynamicComponent instead, which can eliminate the warning. Additionally, both helpers are private.
I'm wondering if there is a more reasonable approach to explicitly use a user-provided layout.

[brc-dd]

Hmm, yeah hard to do this without resolveComponent/resolveDynamicComponent. Other alternative I see is doing something like this https://github.com/vueform/vueform/blob/main/src/utils/isComponentRegistered.js#L3, but that will need getCurrentInstance 🫠

We can go back to not supporting the default theme names it if there are issues with the vapor implementation.

[shigma]

resolveComponent is actually a public API, not private. You can see it in the official docs.