feat: Parse nested element selector

[nzakas]

This adds support for parsing most nested element selectors:

div {
	a {}
	a.foo {}
	a[href] {}
	a, b {}
	a > b {}
	a + b {}
}

The only pattern it doesn't currently cover is when a pseudoclass or pseudoelement is immediately attached to the element, like this:

div {
    a:hover {}
}

That's because a:hover looks like a property and I haven't yet figured out an efficient way to discover whether it's a property or a selector.

refs #33

[coveralls]

Pull Request Test Coverage Report for Build 15310219514

Details

• 27 of 27 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 98.603%

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Totals
Change from base Build 15302932740:	0.003%
Covered Lines:	11738
Relevant Lines:	11870

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💛 - Coveralls

[nzakas]

I needed to add a new parameter because previously isStyleBlock meant it could only contain declarations, which is still true for at-rules but not for regular rules.

[snitin315]

LGTM, thanks!

Leaving it open for 2nd review

[fasttime]

LGTM, thanks!