userdiff: expand detected chunk headers for css

The regex used for the CSS builtin diff driver in git is only
able to show chunk headers for lines that start with a number,
a letter or an underscore.

However, the regex fails to detect classes (starts with a .), ids
(starts with a #), :root and attribute-value based selectors (for
example [class*="col-"]), as well as @based block-level statements
like @page,@keyframes and @media since all of them, start with a
special character.

Allow the selectors and block level statements to begin with these
special characters.

Signed-off-by: Sohom Datta <sohom.datta@learner.manipal.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>