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/editor/src/test/editor_test_context.rs

@@ -6,8 +6,8 @@ use collections::BTreeMap;
 use futures::Future;
 use git::diff::DiffHunkStatus;
 use gpui::{
-    AnyWindowHandle, AppContext, Keystroke, ModelContext, Pixels, Point, View, ViewContext,
-    VisualTestContext, WindowHandle,
+    prelude::*, AnyWindowHandle, AppContext, Keystroke, ModelContext, Pixels, Point, View,
+    ViewContext, VisualTestContext, WindowHandle,
 };
 use itertools::Itertools;
 use language::{Buffer, BufferSnapshot, LanguageRegistry};
@@ -23,8 +23,6 @@ use std::{
         Arc,
     },
 };
-
-use ui::Context;
 use util::{
     assert_set_eq,
     test::{generate_marked_text, marked_text_ranges},