fix: parse innerHTML of an SVG element as SVG (#326)

[jonasporto]

Fixes #326

Problem

In an HTML document, setting innerHTML on an SVG element parses the markup as if it were HTML content: the /> of a self-closing tag is ignored, so siblings end up nested, and the new children are HTMLElements in the XHTML namespace.

import { parseHTML } from 'linkedom';
const { document } = parseHTML('<!doctype html><html><body><svg></svg></body></html>');
const svg = document.querySelector('svg');

svg.innerHTML = '<circle r="1"/><circle r="2"/>';

svg.innerHTML;                       // '<circle r="1"><circle r="2"></circle></circle>'
svg.children.length;                 // 1   (browser: 2)
svg.firstChild.namespaceURI;         // 'http://www.w3.org/1999/xhtml'   (browser: SVG)

Parsing the same markup as part of a document (<body><svg><circle/><circle/></svg>) is already right: htmlparser2 enters its foreign context when it sees <svg>, and parseFromString creates SVG elements under it. The fragment passed to innerHTML has no <svg> around it, so neither happens.

Reproduced on 0.18.12 and on main (3fcf145).

Fix

setInnerHtml (esm/mixin/inner-html.js): when the element is an SVG element in an HTML document, the markup is parsed wrapped in <svg>…</svg> and the wrapper's children are used. This reuses the existing SVG path of parseFromString (no change there, so it does not overlap with #330), and htmlparser2's own foreign-context handling closes />.

The parser points ownerSVGElement of what it creates at the temporary wrapper, so those are set to the element itself when it is an <svg>, or to its ownerSVGElement otherwise.

HTML/XML/SVG documents and non-SVG elements take the same path as before. CJS regenerated with npm run cjs.

Tests

Added to test/svg/element.js: siblings stay siblings, the children are SVGElements with the right ownerSVGElement, the serialization, and innerHTML on a nested <g>. The new test fails without the change (Expected: 2 / Got instead: 1).

npm test passes. Coverage for mixin/inner-html.js is 100%. The overall function figure (99.75%, from interface/element.js at 97.01%) is the same on main without this change.