Move editor into its own crate

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/editor/src/test.rs

@@ -0,0 +1,39 @@
+use gpui::{Entity, ModelHandle};
+use smol::channel;
+use std::marker::PhantomData;
+
+pub fn sample_text(rows: usize, cols: usize) -> String {
+    let mut text = String::new();
+    for row in 0..rows {
+        let c: char = ('a' as u32 + row as u32) as u8 as char;
+        let mut line = c.to_string().repeat(cols);
+        if row < rows - 1 {
+            line.push('\n');
+        }
+        text += &line;
+    }
+    text
+}
+
+pub struct Observer<T>(PhantomData<T>);
+
+impl<T: 'static> Entity for Observer<T> {
+    type Event = ();
+}
+
+impl<T: Entity> Observer<T> {
+    pub fn new(
+        handle: &ModelHandle<T>,
+        cx: &mut gpui::TestAppContext,
+    ) -> (ModelHandle<Self>, channel::Receiver<()>) {
+        let (notify_tx, notify_rx) = channel::unbounded();
+        let observer = cx.add_model(|cx| {
+            cx.observe(handle, move |_, _, _| {
+                let _ = notify_tx.try_send(());
+            })
+            .detach();
+            Observer(PhantomData)
+        });
+        (observer, notify_rx)
+    }
+}