Change internal page data structure to use new front matter schema traits

[caendesilva]

This PR adds schemas as described in #258. These schemas define the public facing data shared to the views, intended to be used by the page templates. For example metadata and SEO information.

The schemas are defined using PHP traits, that also hold the actual data, and may also construct them.

trait BlogPostSchema
{
    public string $title;
    public ?string $description = null;
    public ?string $category = null;
    public ?DateString $date = null;
    public ?Author $author = null;
    public ?Image $image = null;
}

class MarkdownPost
{
    use FrontMatter\BlogPostSchema;
}

Another benefit of this is that we are also less reliant on magic methods and properties. Those offered through the front matter object would only really be needed for ad-hoc data added by the user.

There are more schemas to add than what will be added in this PR. Such as documentation pages and image objects. Everything that can be supplied as front matter should be covered.