Mainline GPUI2 UI work (#3062)

This PR mainlines the current state of new GPUI2-based UI from the
`gpui2-ui` branch.

Release Notes:

- N/A

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>
Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>
Co-authored-by: Nate <nate@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>

crates/ui/src/components/editor.rs

@@ -0,0 +1,25 @@
+use std::marker::PhantomData;
+
+use crate::prelude::*;
+use crate::{Buffer, Toolbar};
+
+#[derive(Element)]
+struct Editor<V: 'static> {
+    view_type: PhantomData<V>,
+    toolbar: Toolbar,
+    buffer: Buffer<V>,
+}
+
+impl<V: 'static> Editor<V> {
+    pub fn new(toolbar: Toolbar, buffer: Buffer<V>) -> Self {
+        Self {
+            view_type: PhantomData,
+            toolbar,
+            buffer,
+        }
+    }
+
+    fn render(&mut self, _: &mut V, cx: &mut ViewContext<V>) -> impl IntoElement<V> {
+        div().child(self.toolbar.clone())
+    }
+}