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/elements/stack.rs

@@ -1,31 +0,0 @@
-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()
-}