switch to a Document trait and introduce Script/Head/Style/Meta components

[ealmloff]

This PR adds support for a few special elements in dioxus that render into the head:

• Title
• Meta
• head::Link
• Script
• Style

Each of these components can be used to add extra information to the head of the page. For example, you can use the Title component to set the title of the page, or the Meta component to add extra metadata to the page.

Limitations

Components that render into the head of the page do have a few key limitations:

• With the exception of the Title component, all components that render into the head cannot be modified after the first time they are rendered.
• Components that render into the head will not be removed even after the component is removed from the tree.

Example

# use dioxus::prelude::*;
fn RedirectToDioxusHomepageWithoutJS() -> Element {
    rsx! {
        // You can use the meta component to render a meta tag into the head of the page
        // This meta tag will redirect the user to the dioxuslabs homepage in 10 seconds
        Meta {
            http_equiv: "refresh",
            content: "10;url=https://dioxuslabs.com",
        }
    }
}

Fullstack Rendering

Head components are compatible with fullstack rendering, but only head components that are rendered in the initial render (before suspense boundaries resolve) will be rendered into the head.

If you have any important metadata that you want to render into the head, it needs to be rendered outside of any pending suspense boundaries.

# use dioxus::prelude::*;
# #[component]
# fn LoadData(children: Element) -> Element { unimplemented!() }
fn App() -> Element {
    rsx! {
        // This will render in SSR
        Title { "My Page" }
        SuspenseBoundary {
            fallback: |_| rsx! { "Loading..." },
            LoadData {
                // This will only be rendered on the client after hydration so it may not be visible to search engines
                Meta { name: "description", content: "My Page" }
            }
        }
    }
}

Closes #1169

TODO:

• Web
• Desktop
• Liveview
• SSR
• Fullstack
• Docs