Fix parsing CSS selectors which contain commas

[westonruter]

I found an issue where a CSS selectors would fail to parse correctly when they contain commas. For example:

.widget:not(.widget_text, .jetpack_widget_social_icons) ul {
    list-style: none;
    margin: 0;
    padding: 0;
}

There should only be one selector here: .widget:not(.widget_text, .jetpack_widget_social_icons) ul. Instead, the parser incorrectly extracts two:

• .widget:not(.widget_text
• .jetpack_widget_social_icons) ul

The fix is to add placeholders for expressions contained in parentheses or brackets prior to splitting, and then replace the placeholders with their original values on the split selectors.

This PR updates the PHP-CSS-Parser to the state of the library with this upstream PR merged: MyIntervals/PHP-CSS-Parser#138

This PR also fixes a PHPCS complaint regarding preg_quote() not having an explicit $delimiter set.

[amedina]

LGTM. Shipt it!