gpui: Add shadow_xs, shadow_2xs and fix shadow values to match Tailwind CSS (#33361)

Release Notes:

- N/A

---

https://tailwindcss.com/docs/box-shadow

| name | value |
| -- | -- |
| shadow-2xs | box-shadow: var(--shadow-2xs); /* 0 1px rgb(0 0 0 / 0.05)
*/ |
| shadow-xs | box-shadow: var(--shadow-xs); /* 0 1px 2px 0 rgb(0 0 0 /
0.05) */ |
| shadow-sm | box-shadow: var(--shadow-sm); /* 0 1px 3px 0 rgb(0 0 0 /
0.1), 0 1px 2px -1px rgb(0 0 0 / 0.1) */ |
| shadow-md | box-shadow: var(--shadow-md); /* 0 4px 6px -1px rgb(0 0 0
/ 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1) */ |
| shadow-lg | box-shadow: var(--shadow-lg); /* 0 10px 15px -3px rgb(0 0
0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1) */ |
| shadow-xl | box-shadow: var(--shadow-xl); /* 0 20px 25px -5px rgb(0 0
0 / 0.1), 0 8px 10px -6px rgb(0 0 0 / 0.1) */ |
| shadow-2xl | box-shadow: var(--shadow-2xl); /* 0 25px 50px -12px rgb(0
0 0 / 0.25) */ |

## Before

<img width="1112" alt="SCR-20250625-nnxn"
src="https://github.com/user-attachments/assets/3bd44938-5de8-4d67-b323-c444b023a4b6"
/>

## After

<img width="1112" alt="SCR-20250625-nnrt"
src="https://github.com/user-attachments/assets/a5bf2401-f808-4712-9cc6-299f530f9165"
/>

crates/repl/src/notebook/cell.rs

@@ -656,7 +656,7 @@ impl Render for CodeCell {
                                             // .bg(cx.theme().colors().editor_background)
                                             // .border(px(1.))
                                             // .border_color(cx.theme().colors().border)
-                                            // .shadow_sm()
+                                            // .shadow_xs()
                                             .children(content)
                                     },
                                 ))),