Implement support for An+B microsyntax

[kasperisager]

Pseudo-class selectors currently only support values that are either selectors or lists of selectors:

alfa/packages/alfa-css/src/types.ts

Line 109 in 27e4ccc

readonly value: Selector | Array<Selector> | null;

However, some functional pseudo-class selectors make use of the An+B microsyntax (https://www.w3.org/TR/css-syntax/#anb), for example :nth-child(2n+1). To support these kinds of selectors, we first need to implement support for this syntax within the selector parser. The lexer can already lex them:

alfa/packages/alfa-css/test/alphabet.spec.ts

Lines 335 to 349 in 27e4ccc

	test("Can lex an+b values", t => {
	css(t, "2n+4", [
	{
	type: TokenType.Dimension,
	value: 2,
	integer: true,
	unit: "n"
	},
	{
	type: TokenType.Number,
	value: 4,
	integer: true
	}
	]);
	});

[Bugvi-Benjamin-M]

@2biazdk has been working on this for some time but has been caught up with other stuff that has prevented him from making progress lately. We are going to take a look at this soon.

[kasperisager]

Reopening as support will need to be reintroduced following #165.