Allow CSS level 3 selectors for classes and ids in the attributes extension

[xavierlacot]

The CSS level 3 Syntax, which is a Recommendation since 2018, allows much more characters in id and class selectors than the ones currently supported by the "Attributes" extension:

commonmark/src/Extension/Attributes/Util/AttributesHelper.php

Line 26 in 91c2429

private const SINGLE_ATTRIBUTE = '\s*([.#][_a-z0-9-]+|' . RegexHelper::PARTIAL_ATTRIBUTENAME . RegexHelper::PARTIAL_ATTRIBUTEVALUESPEC . ')\s*';

The formal CSS level 3 selectors grammar is detailed here: https://www.w3.org/TR/selectors-3/#w3cselgrammar

This pull request adds a test to check several valid class and id names that are currently failing to be parsed as valid attribute values by the "Attributes" extension, and it proposes a fix for this issue.