Make `termcolor` types public in `rustc_errors`
After https://github.com/rust-lang/rust/pull/114104, `rust-gpu` is unable to create a custom `Emitter` as the bounds have changed to include `WriteColor`.
I was able to work around this by adding `termcolor` as a direct dependency, but I believe this should be exposed as part of `rustc_errors` proper.
See https://github.com/rust-lang/rust/pull/102992 for why `rust-gpu` needs to create a custom emitter.
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