Start integrating Copilot with editor

There's still a bit to do in terms of reusing the previous suggestion
when the prefix matches, but we're getting there.

crates/copilot/src/sign_in.rs

@@ -7,7 +7,7 @@ use gpui::{
 use settings::Settings;
 
 pub fn init(cx: &mut MutableAppContext) {
-    let copilot = Copilot::global(cx);
+    let copilot = Copilot::global(cx).unwrap();
 
     let mut code_verification_window_id = None;
     cx.observe(&copilot, move |copilot, cx| {