Include the sign in the NUMBER token in CSS 2.1

[gsnedders]

From https://lists.w3.org/Archives/Public/www-style/2012May/0063.html:

• RESOLVED: Include the sign in the NUMBER token in CSS 2.1.

This appears to be originally reported in https://lists.w3.org/Archives/Public/www-style/2012Apr/0639.html from @tabatkins and @fantasai.

Fixed in CSS 2.1 errata.

Note that this text is, per WG resolution, to become informative (see #2224 (comment)).

[gsnedders]

https://www.w3.org/Style/css2-updates/REC-CSS2-20110607-errata.html#s.G.1a is further fixes for this.

Note @dbaron filed #987 for that change.