The prefix "bright" will continue to make the color brighter AND/OR
make the typeface bold (depending on how the terminal is configured
and on its capabilities), but the prefix "light" will just make the
color brighter (if the terminal can do that).
The prefix "bright" continues to do the same as what it did before,
to not change the appearance of user-defined syntaxes. And having
an option to change the meaning of "bright"... would be confusing.
This partially fulfills https://savannah.gnu.org/bugs/?58503.
4d10f583e4