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/gpui/examples/shadow.rs

@@ -156,6 +156,10 @@ impl Render for Shadow {
                     .w_full()
                     .children(vec![
                         example("None", Shadow::base()),
+                        // 2Xsmall shadow
+                        example("2X Small", Shadow::base().shadow_2xs()),
+                        // Xsmall shadow
+                        example("Extra Small", Shadow::base().shadow_xs()),
                         // Small shadow
                         example("Small", Shadow::base().shadow_sm()),
                         // Medium shadow