void element list duplicated in 4 places

[dannywillems]

Description

The void element list (area, base, br, col, embed, hr, img, input, link, meta, source, track, wbr) is defined independently in:

1. `crates/ironhtml/src/lib.rs` — untyped `Element::new`
2. `crates/ironhtml-parser/src/dom.rs` — parser `Element::is_void`
3. `crates/ironhtml-parser/src/tree_builder.rs` — tree builder
4. `crates/ironhtml-elements/src/lib.rs` — `const VOID: bool = true` on each type

A rendering bug fix would need to be applied in multiple places. The `ironhtml-elements` crate is the canonical source via `HtmlElement::VOID`, but the runtime crates use string matching instead of referencing the type system.