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/completion/src/completion.rs

@@ -89,10 +89,10 @@ impl LanguageModelCompletionProvider {
 
     pub fn set_active_provider(
         &mut self,
-        provider_name: LanguageModelProviderId,
+        provider_id: LanguageModelProviderId,
         cx: &mut ModelContext<Self>,
     ) {
-        self.active_provider = LanguageModelRegistry::read_global(cx).provider(&provider_name);
+        self.active_provider = LanguageModelRegistry::read_global(cx).provider(&provider_id);
         self.active_model = None;
         cx.notify();
     }