perf: read class attribute directly in selector adapter

[PeronGH]

When matching selectors, getAttributeValue read class through element.classList.value, which creates the classList for every element it checks. This change reads the class attribute node directly:

const getAttributeValue = (element, name) => name === 'class' ?
  (element.getAttributeNode('class')?.value ?? null) : element.getAttribute(name);

Behaviour change (fix)

[class=""] used to match elements that have no class attribute, because classList.value is '' for them. Browsers only match elements that have an empty class attribute. Now linkedom does too. I added a test to test/html/document.js. It covers HTML and SVG elements, removeAttribute, className and classList.

Benchmark

2000 elements, 50 class-heavy selector groups, querySelectorAll run 500 times (Node, 3 runs each):

	time
before	~1045 ms
after	~143 ms