Standardize TabBar start_slot and end_slot elements (#11403)

- Unifies spacing between left and right sides of the tab bar
- Use the default icon color for `end_slot` tools. This should help more
clearly differentiate when forward or backward navigation is disabled
due to the tools on the other side not looking so much like the disabled
navigation arrows.
- Rework the TabBar implementation in `pane.rs` to directly pass in
items to the `start_slot` instead of an unneeded extra horizontal
layout.

Left side:

![CleanShot 2024-05-05 at 11 08
35@2x](https://github.com/zed-industries/zed/assets/1714999/ec80fda5-17ce-4cd4-ae54-8c63dcc79e69)

Right side:

![CleanShot 2024-05-05 at 11 09
04@2x](https://github.com/zed-industries/zed/assets/1714999/0281e462-202f-407b-b6b7-7acbcde9138f)

Release Notes:

- Standardized some Tab Bar UI elements. You many notice some slight
spacing or color changes.

crates/ui/src/components/tab_bar.rs

@@ -107,7 +107,7 @@ impl RenderOnce for TabBar {
                     h_flex()
                         .flex_none()
                         .gap(Spacing::Small.rems(cx))
-                        .px(Spacing::Small.rems(cx))
+                        .px(Spacing::Medium.rems(cx))
                         .border_b()
                         .border_r()
                         .border_color(cx.theme().colors().border)