feat(text/style): detect terminal color capability via $COLORTERM

[rawiriblundell]

Background

`text/style.sh` unconditionally emits ANSI color escape sequences with no check
for whether the terminal actually supports them. `$COLORTERM` is the
de-facto environment variable for advertising true color support:

• `COLORTERM=truecolor` or `COLORTERM=24bit` — terminal supports 24-bit color
• Unset or other value — assume 8/256 color at most

Proposal

Add a function (e.g. `str_has_truecolor`) that tests `$COLORTERM` and the
`$TERM` value to determine whether true color output is safe. Callers can
use it to decide whether to emit 24-bit escapes (via `text_COLOR_FG_rgb` /
`text_COLOR_BG_rgb`) or fall back to 256-color equivalents.

A broader `str_color_depth` function returning 24, 256, 8, or 0 (no color)
may also be worth considering — covering `$NO_COLOR`, `$TERM=dumb`,
non-interactive shells, and piped output.

References

• https://gist.github.com/XVilka/8346728
• https://stackoverflow.com/a/33206814
• https://cubicspot.blogspot.com/2019/05/designing-better-terminal-text-color.html