Handling of doctype entities

[noahbald]

xml5ever/html5ever doesn't support full doctype parsing for entity declarations. We may need to consider a different parser

• https://github.com/RazrFalcon/xmlparser seems nice but we'll need to build the tree ourself and do something about lifetimes
• https://docs.rs/swc_xml_ast/6.0.0/swc_xml_ast/struct.DocumentType.html also seems nice but it's a read-only tree. We'd need to clone the tree into our own data structure. Or we can use swc_xml_parser to parse into our struct

[noahbald]

I was also hoping html5ever might still be usable since it parses entities into comments, however it seems to be unreliable and can drop DTD Entity declarations.
e.g.

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd" [<!ENTITY st1 "opacity:.34;"><!ENTITY st2 "fill:#231F20;">]>
<svg version="1.1" xmlns="http://www.w3.org/2000/svg"></svg>

Given this input svg, the first entity is lost

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<!-- ENTITY st2 "fill:#231F20;" -->
<svg version="1.1" xmlns="http://www.w3.org/2000/svg">
</svg>

This would have been useful in prechecks (#119) but sadly would make it unreliable