inline_completion_button: Put "Eager Preview Mode" menu entry behind a feature flag (#24734)

This PR puts the "Eager Preview Mode" menu entry behind a feature flag
rather than a staff flag.

Currently it defaults to `false` for staff so that it doesn't leak into
any marketing/launch materials.

Folks who want to see it can opt-in to the flag explicitly, for now.

Release Notes:

- N/A

crates/feature_flags/src/feature_flags.rs

@@ -69,6 +69,17 @@ impl FeatureFlag for PredictEditsRateCompletionsFeatureFlag {
     const NAME: &'static str = "predict-edits-rate-completions";
 }
 
+/// A feature flag that controls whether "non eager mode" (holding `alt` to preview) is publicized.
+pub struct PredictEditsNonEagerModeFeatureFlag;
+impl FeatureFlag for PredictEditsNonEagerModeFeatureFlag {
+    const NAME: &'static str = "predict-edits-non-eager-mode";
+
+    fn enabled_for_staff() -> bool {
+        // Don't show to staff so it doesn't leak into media for the launch.
+        false
+    }
+}
+
 pub struct GitUiFeatureFlag;
 impl FeatureFlag for GitUiFeatureFlag {
     const NAME: &'static str = "git-ui";