feat: [#2390] support the :required CSS pseudo-class

[marciomazza]

Description

Resolves #2390

The :required CSS pseudo-class is never matched — SelectorItem.matchPseudoItem
has no required case, so any :required selector silently returns nothing.

document.body.innerHTML = `<input id="a" required><input id="b">`;
document.querySelectorAll(':required').length; // happy-dom: 0 — real browsers: 1

Fix: add a required case mirroring the existing disabled one — it matches when
the element has a required content attribute.

Deviation left on purpose: this also matches <input type="hidden" required>
(the check is 'required' in element, not whether the attribute is applicable to
the element's current type), where browsers do not. The existing :disabled case
is equally loose; tightening both is out of scope here.

Tests

Added 1 test: :required matches <input> / <select> / <textarea> carrying
the attribute and skips an <input> without it. Fails before / passes after.
Full npm test passes.

AI

I used Claude Code to write this, over multiple iterations. I guided and reviewed
it myself at every step.

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Tests

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  • Ran the repro in Chrome 152 (headless): :required matched <input required>, <select required>, <textarea required>, and did not match <input type="hidden" required>. The test asserts the first three.
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