Adjust model selector popover design (#15056)

This PR mostly refines the model selector popover design by formatting
the models names' and adjusting spacing/alignment in the list-related
items. The list component changes could've been made in a separate PR
but it was also very practical to do it here as I was already
in-context. Either way, I'm happy to separate if that's better!

One thing I couldn't necessarily figure out, though, is why the order
changed (e.g., Anthropic at last ). I wonder if that was because of the
separator logic somehow? I'd love guidance here—new to Rust!

| Before | After |
|--------|--------|
| <img width="228" alt="Screenshot 2024-07-23 at 21 02 33"
src="https://github.com/user-attachments/assets/3372c6c9-08dc-4d71-9265-26f015e2dbc2">
| <img width="228" alt="Screenshot 2024-07-23 at 21 01 45"
src="https://github.com/user-attachments/assets/624cc7db-a3d9-48e3-99d7-c29829501130">
|

---

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/ui/src/components/list/list_item.rs

@@ -185,7 +185,7 @@ impl RenderOnce for ListItem {
                     .w_full()
                     .relative()
                     .gap_1()
-                    .px_2()
+                    .px_1p5()
                     .map(|this| match self.spacing {
                         ListItemSpacing::Dense => this,
                         ListItemSpacing::Sparse => this.py_1(),

crates/ui/src/components/list/list_separator.rs

@@ -8,7 +8,7 @@ impl RenderOnce for ListSeparator {
         div()
             .h_px()
             .w_full()
-            .my_1()
+            .my_1p5()
             .bg(cx.theme().colors().border_variant)
     }
 }

crates/ui/src/components/list/list_sub_header.rs

@@ -39,7 +39,7 @@ impl Selectable for ListSubHeader {
 
 impl RenderOnce for ListSubHeader {
     fn render(self, cx: &mut WindowContext) -> impl IntoElement {
-        h_flex().flex_1().w_full().relative().py_1().child(
+        h_flex().flex_1().w_full().relative().pb_1().px_0p5().child(
             div()
                 .h_6()
                 .when(self.inset, |this| this.px_2())