Note: My copyright assignment paperwork is out of date, but I've
already sent a message to assign@ to get it updated. Thus, these
patches won't be able to merge right away. However, I wanted to start
the review process now so that there's plenty of time for
back-and-forth before Emacs 28.1 is released.

With the administrative issues out of the way... these patches provide
support for "bright" ANSI colors (SGR 90-97 and 100-107 for foreground
and background, respectively)[1]. Most of the complexity here is due
to the new defcustoms `*-bold-is-bright'. Enabling this results in
ANSI "bold" text (SGR 1) to be rendered in the bright color palette
(as well as being bold). This is a pretty common option in terminal
emulators; all the ones I looked at[2] support it, and it's often the
default behavior. For me, the main benefit of this option is so I can
easily match the color palettes between Emacs and my terminal
emulator.

I've split this into two patches, one for 'ansi-color' and one for
'term-mode'. Despite the similarity in functionality, the
implementations are pretty different. It might be nice if they could
be unified somehow, but that may be more trouble than it's worth...

- Jim

[1] https://en.wikipedia.org/wiki/ANSI_escape_code#SGR_(Select_Graphic_Rendition)_parameters
[2] gnome-terminal, alacritty, and PuTTY