Allocate styles inside internal pages and schemes

[Ambrevar]

From https://github.com/atlas-engineer/nyxt/pull/2536#discussion_r961925124.

Currently history-tree-mode exists for the sole purpose of defining a style. Seems overkill: having styles in internal pages seems more elegant.

[aartaka]

And here we have first use-case for user-funcallable-class!

[Ambrevar]

How so?

[aartaka]

Well, styles should be configurable, and thus we need to configure command and internal-page objects too, which is a use-case for their user class—user-funcallable-class.

[aartaka]

But the way, I'm not sure about schemes there: those may return non-HTML things, so I'm cautious about implying a style (and, thus, the structure of the HTML document) for those...