Introduce staff-only inline completion provider (#21739)

Release Notes:

- N/A

---------

Co-authored-by: Thorsten Ball <mrnugget@gmail.com>
Co-authored-by: Bennet <bennet@zed.dev>
Co-authored-by: Thorsten <thorsten@zed.dev>

crates/language/src/language_settings.rs

@@ -197,6 +197,7 @@ pub enum InlineCompletionProvider {
     #[default]
     Copilot,
     Supermaven,
+    Zeta,
 }
 
 /// The settings for inline completions, such as [GitHub Copilot](https://github.com/features/copilot)