uistyle: Support rendering of hex colors

uistyle: Support rendering of hex colors

Hex colors are a relatively new addition to IRC, as defined in
<https://modern.ircdocs.horse/formatting.html#hex-color>.

Note that there are known issues with prioritization in the theme
engine (i.e., setting the foreground explicitly in the stylesheet
will always override hex colors), which will be fixed in a follow-up
commit.