ui2: Reorganize components (#3228)

This PR reorganizes the components in the `ui2` crate.

The distinction between "elements" and "components" is now gone, with
all of the reusable components living under `components/`.

The components that we built while prototyping but will eventually live
in other crates currently reside in the `to_extract/` module.

Release Notes:

- N/A

crates/ui2/src/components/stack.rs

@@ -0,0 +1,31 @@
+use gpui2::{div, Div};
+
+use crate::prelude::*;
+
+pub trait Stack: Styled + Sized {
+    /// Horizontally stacks elements.
+    fn h_stack(self) -> Self {
+        self.flex().flex_row().items_center()
+    }
+
+    /// Vertically stacks elements.
+    fn v_stack(self) -> Self {
+        self.flex().flex_col()
+    }
+}
+
+impl<V: 'static> Stack for Div<V> {}
+
+/// Horizontally stacks elements.
+///
+/// Sets `flex()`, `flex_row()`, `items_center()`
+pub fn h_stack<V: 'static>() -> Div<V> {
+    div().h_stack()
+}
+
+/// Vertically stacks elements.
+///
+/// Sets `flex()`, `flex_col()`
+pub fn v_stack<V: 'static>() -> Div<V> {
+    div().v_stack()
+}