Add support for HTMLButtonElement.

[fsoikin]

When writing some tests, I noticed this:

const el = document.createElement("button")
el.setAttribute("disabled", "")
console.log(el.disabled)  // <-- Prints "undefined"

In a real browser, the last line would print out true. Checked in Chrome and FireFox.

After digging a bit, I realized that happy-dom uses the generic HTMLElement to represent a <button>, which doesn't translate between standard button attributes and properties.

This PR adds a HTMLButtonElement class and makes it translate some (not all) of the properties - specifically, value, disabled, and type.

[capricorn86]

Looks good! ⭐ Just some minor things.

[capricorn86]

Thank you for contributing @fsoikin! 🙂

Sorry for not looking into this PR earlier. There has been a lot in my private and work life.

[capricorn86]

Great work ⭐