add requires_theme to site.collection

[kjaymiller]

Some custom collections will have associated themes with them. We need to provide these themes by default.

Perhaps a "required_themes" could be for included themes loaders.

This way the custom collection or page object can include a loader that would be added to the site when the @site.collection|page is added.

Example

# site.py
# in the Site class
    def collection(self, Collection: type[Collection]) -> Collection:
        _Collection = Collection()
        ###### NEW####
        for theme in getattr(_Collection, "required_themes", []):
            site.engine.loaders.insert(theme, 0)
         ##############
        plugins = [*self.plugins, *getattr(_Collection, "plugins", [])]
        ...

[kjaymiller]

Not sure why I acted like site doesn't have a way to do this already

Better Example:

Example

# site.py
# in the Site class
    def collection(self, Collection: type[Collection]) -> Collection:
        _Collection = Collection()
        ###### NEW####
            self.register_themes(getattr(_Collection, "required_themes", [])
         ##############
        plugins = [*self.plugins, *getattr(_Collection, "plugins", [])]
        ...

We could also just call them themes. It's a little odd that we would call them just themes where with plugins we're passing the site's plugins in at rendering time. (At least that's the desired outcome)

It seems plugins aren't behaving as desired.