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/editor/src/editor.rs

@@ -8715,7 +8715,7 @@ impl Editor {
                 h_flex()
                     .bg(cx.theme().colors().editor_background)
                     .border(BORDER_WIDTH)
-                    .shadow_sm()
+                    .shadow_xs()
                     .border_color(cx.theme().colors().border)
                     .rounded_l_lg()
                     .when(line_count > 1, |el| el.rounded_br_lg())
@@ -8915,7 +8915,7 @@ impl Editor {
             .border_1()
             .bg(Self::edit_prediction_line_popover_bg_color(cx))
             .border_color(Self::edit_prediction_callout_popover_border_color(cx))
-            .shadow_sm()
+            .shadow_xs()
             .when(!has_keybind, |el| {
                 let status_colors = cx.theme().status();