Accept CSS numbers without a leading zero in rgb/rgba/hsl/hsla

[fukayatsu]

RE_COLOUR_NUMERIC / RE_COLOUR_NUMERIC_ALPHA use \d+(\.\d+)? for each
numeric component, which requires at least one digit before the decimal
point. CSS Values & Units allows <number> to omit the integer part
([0-9]* '.' [0-9]+), so values like rgba(0,0,0,.1) are not recognised
as colours.

The user-visible effect is that shorthand expansion silently drops the
colour. For example:

parser = CssParser::Parser.new
parser.add_block!(".a { border: 1px solid rgba(0,0,0,.1); }")
parser.each_rule_set { |rs, _| rs.expand_shorthand! }
# => border-top-width / -style are kept, border-top-color is gone.

The same shorthand with rgba(0,0,0,0.1) works correctly. background:
is affected the same way (background-color is dropped).

This bites Premailer users in the wild because Dart Sass's compressed
output strips leading zeros, so .1 is what the email inliner actually
receives.

This PR replaces \d+(\.\d+)? with (?:\d*\.)?\d+ in both numeric
colour regexps. The new pattern accepts 1, 1.5, and .5 while still
rejecting bare 1. (also invalid per spec).

Tests:

• Positive cases added to RE_COLOUR (.1-style alpha for rgba/hsla).
• Shorthand-expansion regression tests for both border: and
  background: covering rgba(0,0,0,.1).

Pre-Merge Checklist

• CHANGELOG.md updated with short summary

[grosser]

thx! 2.2.0