gpui: Don't export named Context from prelude (#21869)

This PR updates the `gpui::prelude` to not export the `Context` trait
named.

This prevents some naming clashes in downstream consumers.

Release Notes:

- N/A

crates/project_panel/src/project_panel.rs

@@ -4262,7 +4262,6 @@ mod tests {
     use serde_json::json;
     use settings::SettingsStore;
     use std::path::{Path, PathBuf};
-    use ui::Context;
     use workspace::{
         item::{Item, ProjectItem},
         register_project_item, AppState,