Fix #12280: replace difference crate with similar (#12282)

Fixes #12280.

# Description

This removes the dependency on the `difference` crate, which is
unmaintained, for `nu-plugin-test-support`. The `similar` crate
(Apache-2.0) is used instead, which is a bit larger and more complex,
but still suitable for a dev dep for tests. Also switched to use
`crossterm` for colors, since `similar` doesn't come with any terminal
pretty printing functionality.

# User-Facing Changes

None - output should be identical.

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

crates/nu-plugin-test-support/src/lib.rs

@@ -63,6 +63,7 @@
 //! # test_lowercase().unwrap();
 //! ```
 
+mod diff;
 mod fake_persistent_plugin;
 mod fake_register;
 mod plugin_test;