Add some CSS level 3 structural pseudo-classes

[ix]

This PR adds the following CSS level 3 structural pseudo-classes to yesod-test:

• :first-child
• :last-child
• :nth-child (with an implementation of the 'an + b' syntax)

... I've decided to implement the remaining set of pseudo-classes, so have marked this as a draft in the meantime

Reference material

These pseudo-classes facilitate easier matching of sequential elements and are particularly useful when inspecting lists or tables.

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• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddocks for new, public APIs

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• Update the Changelog.md file with a link to your PR
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[jezen]

This looks very cool!

[ix]

Thanks! I'm aiming to implement the remainder of the Level 3 structural pseudo-classes within the next few days and I'll also add more test cases in main.hs against the Firefox implementation (using the following function).

function runQuery (s, e) {
  return new DOMParser().parseFromString(s, "text/html").querySelectorAll(e);
}

Once that's done I'll unmark this as draft 👍