Allow dereferencing AnsiString as underlying string

[richardhozak]

Hello, I've been in process of converting https://github.com/ogham/exa from rust-ansi-term to nu-ansi-term and there is one api that exa needs that was lost during the refactoring from rust-ansi-term to nu-ansi-term.

exa needs the underlying raw string to calculate widths, previously this api was present in a form, where AnsiString could be dereferenced into Cow<'a, str>, directly and the to str, which was then used.

This adds the minimal required code, it just returns &str which is the minimum needed.

I tried to workaround this with some other api but could not found any in nu-ansi-term and to_string would require allocation which seemed unnecessary as the underlying view can be returned directly.

[sholderbach]

I removed the Deref implementation in 1df156e because we were running into issues with clippy suggesting to remove to_string() calls that were actually necessary to apply the styling.
Sadly I wasn't able to cook up a working minimal reproducible example to check with the clippy folks if that is a false positive or if the original implementation was unsound.

[richardhozak]

I see, but this should not trigger the lint I think, the problem seems to be that it was implemented as Deref, this is just ordinary getter.

[sholderbach]

Yeah your PR looks good to me and handles as expected in playing with it. I just wanted to keep the historical record around, for those coming from the original ansi_term crate