Support plaintext, log-friendly, format for traceback

[s-fleck]

rlang tracebacks look great in the CLI, but they are not very log friendly.

When logging it's relatively hard to get a nice readable output. paste(cli::ansi_strip(format([trace](rlang::trace_back()))), collapse = "\n") kinda works and is good enough, but it would be nice to have some argument in the format method to just switch of all color output.

[lionel-]

You can turn off ANSI escapes with an environment variable or R option: https://cli.r-lib.org/reference/num_ansi_colors.html#details