Switch from termcolor to anstream

[hanna-kruppe]

Tested by running the simple example with various flags and environment variables, writing to a tty/pipe/file. I have not tested Windows console support but anstream explicitly supports both legacy wincon and the new ANSI capabilities, and I guess people are exercising that support via clap.

I also tested the new MSRV by running these commands (1.64 does not work):

rm Cargo.lock
CARGO_RESOLVER_SOMETHING_LIKE_PRECEDENCE=something-like-rust-version cargo +nightly -Zmsrv-policy check
cargo +1.65 test

Fix #42

[hanna-kruppe]

Note: this achieves de-duplication for everyone using clap v4.4.6 or later. Earlier clap versions (for MSRV) work, they might just pull in two versions of anstream. That's alright for me, but raises the question of what happens if and when anstream has another semver-breaking release and clap adopts that. This is purely hypothetical at this point, but I think I'd prefer libtest-mimic to follow latest clap as much as feasible. Alternatively, if the breaking changes don't affect the API subset used in libtest-mimic, it could support a range of anstream versions, just like some crates depend on itertools = ">= 0.10, <= 0.12" or similar.