Extract panel resize code from sidebar into Resizable element

Make resizable work in vertical axis
Make dock resizable
Have dock preserve size based on Anchor position
Make pane buttons work more correctly in pathological cases
Sync status bar dock button with dock visibility/anchor position

Co-Authored-By Mikayla Maki <mikayla@zed.dev>

crates/workspace/src/sidebar.rs

@@ -189,13 +189,15 @@ impl View for Sidebar {
     fn render(&mut self, cx: &mut RenderContext<Self>) -> ElementBox {
         if let Some(active_item) = self.active_item() {
             enum ResizeHandleTag {}
+            let style = &cx.global::<Settings>().theme.workspace.sidebar;
             ChildView::new(active_item.to_any())
+                .contained()
+                .with_style(style.container)
                 .with_resize_handle::<ResizeHandleTag, _>(
                     self.sidebar_side as usize,
                     self.sidebar_side.to_resizable_side(),
-                    // TODO: Expose both of these constants in the theme
                     4.,
-                    260.,
+                    style.initial_size,
                     cx,
                 )
                 .boxed()