Switch from ansi_term to yansi (#288)

ansi_term is not actively maintained anymore (see
https://rustsec.org/advisories/RUSTSEC-2021-0139). Replace it with a
suitable alternative.

I looked at termcolor, owo-colors and yansi and picked yansi:

- It is very simple
- It does not do terminal color support checking (we already do that in
  tealdeer)
- Its color enum type is almost identical to the one from ansi_term, so
  migrating is easy

The only change from a config API point of view is that "purple" is now
renamed to "magenta", but "purple" still works.

Cargo.toml

@@ -20,7 +20,6 @@ name = "tldr"
 path = "src/main.rs"
 
 [dependencies]
-ansi_term = "0.12.0"
 anyhow = "1"
 app_dirs = { version = "2", package = "app_dirs2" }
 atty = "0.2"
@@ -32,6 +31,7 @@ serde = "1.0.21"
 serde_derive = "1.0.21"
 toml = "0.5.1"
 walkdir = "2.0.1"
+yansi = "0.5"
 zip = { version = "0.6", default-features = false, features = ["deflate"] }
 
 [target.'cfg(not(windows))'.dependencies]