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>

styles/src/styleTree/workspace.ts

@@ -37,11 +37,14 @@ export default function workspace(theme: Theme) {
       },
       cursor: "Arrow",
     },
-    sidebarResizeHandle: {
-      background: border(theme, "primary").color,
-      padding: {
-        left: 1,
-      },
+    sidebar: {
+      initialSize: 240,
+      border: {
+        color: border(theme, "primary").color,
+        width: 1,
+        left: true,
+        right: true,
+      }
     },
     paneDivider: {
       color: border(theme, "secondary").color,
@@ -157,6 +160,8 @@ export default function workspace(theme: Theme) {
       margin: { right: 10, bottom: 10 },
     },
     dock: {
+      initialSizeRight: 240,
+      initialSizeBottom: 360,
       wash_color: withOpacity(theme.backgroundColor[500].base, 0.5),
       flex: 0.5,
       panel: {