LibWeb: Implement CSS color parsing from Tokens

This was broken when we switched away from using StringStyleValues.
While I was at it, I have implemented hsl/a() and the percentage
syntax for rgb/a().

As a bonus, added `colors.html` as a test page for the various CSS
color syntaxes, since nothing was testing rgb() or rgba() before.

Much of the parsing code in LibGFX/Color.h seems to be centered
around CSS color values, but this is not used by the new Parser.
(And can't be used, because it requires a String value and we have
a list of Tokens of some kind instead.) Maybe that should be removed
from there when the new CSS parser is operational.

Userland/Libraries/LibWeb/CSS/Parser/Token.h

@@ -76,6 +76,8 @@ public:
         return m_value.string_view();
     }
 
+    bool is(NumberType number_type) const { return is(Token::Type::Number) && m_number_type == number_type; }
+
     int integer() const
     {
         VERIFY(m_type == Type::Number && m_number_type == NumberType::Integer);