[css-syntax] Revert (parts of) "Revert <unicode-range> changes from CSS 2.1"

This reverts parts of commit 23590701e57879cfb0ba67e47524c2f2b889fe74.

Namely:

* The tokens have a pair of integers, rather than strings or "a range".
* Keep the "Consume a unicode-range token" section as in earlier EDs.
* Remove the "Set the unicode-range’s range" section: the tokenizer does
no error handling / normalization beyond parsing hex digits into
integers.