Add JSX/XML-like tag syntax to the rsx! macro

[nicoburns]

Summary

Stacked on #5883. Adds a JSX/XML-like tag syntax to rsx!, freely mixable with the existing block syntax (similar to dioxus-html-macro, but integrated into the one macro). A < token switches BodyNode::parse into tag mode:

rsx! {
    <div class="container" onclick={move |_| info!("clicked")}>
        <h1>"Hello, {name}!"</h1>
        <img src="image.png" />
        <MyComponent prop="value" {..props}>"children"</MyComponent>

        // both syntaxes mix freely in either direction
        p { class: "regular", "regular child" }
        for item in items {
            <span>"{item}"</span>
        }
    </div>
}

The new parser (packages/rsx/src/jsx.rs) produces the same Element/Component AST as the block syntax, so templates, hot-reloading, attribute merging, and diagnostics behave identically. Element-vs-component tag names, string-literal text nodes, key, shorthand attributes (<div class>), custom/dashed attributes (data-foo="1", "custom-attr"="x"), web components (<my-widget />), component paths/generics (<some::Outlet<R>>, closing tag generics optional), and {..spread} all follow the existing rsx rules. Attribute values must be literals or braced expressions (attr={expr}).

Breaking change (dioxus-rsx): the brace: Option<Brace> field of Element and Component is replaced by delimiter: NodeDelimiter:

pub enum NodeDelimiter {
    Brace(Brace),        // div { }
    Tag(TagDelimiter),   // <div></div> / <div />: the real `<`, `/`, `>` and closing tag tokens
    Missing,             // div   (was `brace: None`)
}

BodyNode::first_token_span() returns the < for tags (BodyNode::span() still points at the name).

Error handling:

• Tags that are still being typed are parsed as far as possible and reported as diagnostics instead of failing the macro, like the block syntax: unterminated open tags (<di, <div cl, <div class=), missing closing tags, and attribute value expressions that don't parse yet (class={foo.}, kept as raw tokens). An unterminated or unclosed tag does not swallow its siblings or its parent's closing tag.
• A closing tag that matches neither the open tag nor one of its parents is still a hard error (closing tag `</dvi>` does not match opening tag `<div>` ).
• Unquoted text gets a dedicated error (text in a tag must be a quoted string literal). A single word directly before a tag (<h1>Hello</h1>) is ambiguous with an element/component that is being typed, so it expands with a diagnostic that suggests quoting it.
• Fragments (<>...</>) are rejected with a dedicated error.

Supporting changes:

• Element::from_parts / Component::from_parts extracted from the existing Parse impls so both syntaxes share the merge/validation logic.
• ExprNode::parse ends a {expr} or match node at a following <, so {children}</div> and match x { .. } </ul> parse. This is limited to nodes (which can never be comparisons); PartialExpr::parse is unchanged, so attribute values like hidden: {a} < b still parse as expressions.
• autofmt: dx fmt rewrites tag syntax into the block syntax in this PR (Preserve JSX tag syntax in autofmt #5731 makes it preserve tags). The only autofmt change here is glue: the BodyDelimiters that fix(autofmt): stop dropping comments on attributes, spreads, control flow and empty macros #5883 introduced are built from a NodeDelimiter, mapping the tag tokens onto the open/close positions, so comments before/after tags, on attributes, and before closing tags survive the rewrite.

Tests: parser unit tests in jsx.rs (elements, components, generics, web components, spreads, shorthand, mixing, incomplete tags, unquoted text, error cases, codegen), SSR rendering tests in packages/ssr/tests/jsx_syntax.rs, a hot-reload test, and an autofmt test for comment preservation (tests/wrong/jsx-comments*).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/99eedb36d44345dc8ce0f3ed5c460bd6
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/99eedb36d44345dc8ce0f3ed5c460bd6?variant=devin-insiders
Requested by: @nicoburns

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