Added 'first-child' pseudo-selector test.

This is pretty much just an small rewrite of the 'css-lastchild.js' feature test already present on Modernizr to allow for the detection of browser support for the `:first-child` pseudo-selector. I am pretty sure that every browser that supports `:last-child` also supports `:first-child` (and vice-versa), and as such including this may sound a bit redundant, but if I am using `:first-child` anywhere I'd rather write

```css
.firstchild {
    /* (...) */
}
.no-firstchild {
    /* (...) */
}
```

than `.lastchild` and `.no-lastchild`.

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