instead, allow the user to set it as a field.

this fixes a bug where leak printing and error printing would run tty
config detection for stderr, and then emit a log, which is not necessary
going to print to stderr.

however, the nice defaults are gone; the user must explicitly assign the
tty_config field during initialization or else the logging will not have
color.

related: https://github.com/ziglang/zig/issues/24510