Revise "Hide/Show Inline Completions" menu (#23808)

> **Note:** https://github.com/zed-industries/zed/pull/23813 should be
merged first!

@nathansobo and I paired on revising this menu, including adding the
"Predict Edits at Cursor" menu item (to make the keyboard shortcut more
discoverable; clicking it makes the inline edits show up, as shown in
the second screenshot) and switching from "Hide/Show" language to
checkboxes.

## Before
<img width="282" alt="Screenshot 2025-01-28 at 4 51 37 PM"
src="https://github.com/user-attachments/assets/309c82c1-8fb5-44db-950e-1a8789a63993"
/>

## After
<img width="1138" alt="Screenshot 2025-01-28 at 4 50 05 PM"
src="https://github.com/user-attachments/assets/302a126c-9389-42a4-bb7d-2896bce859e7"
/>

We also switched to use `SharedString` in more places, where it made
more sense.

@danilo-leal This isn't necessarily *exactly* what we want, but we were
pairing and decided to get it in a state where we can actually try it
out and tweak from here.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Marshall Bowers <git@maxdeviant.com>

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

@@ -49,7 +49,7 @@ impl RenderOnce for ListSubHeader {
             .px(DynamicSpacing::Base02.rems(cx))
             .child(
                 div()
-                    .h_6()
+                    .h_5()
                     .when(self.inset, |this| this.px_2())
                     .when(self.selected, |this| {
                         this.bg(cx.theme().colors().ghost_element_selected)
@@ -70,7 +70,11 @@ impl RenderOnce for ListSubHeader {
                                     Icon::new(i).color(Color::Muted).size(IconSize::Small)
                                 }),
                             )
-                            .child(Label::new(self.label.clone()).color(Color::Muted)),
+                            .child(
+                                Label::new(self.label.clone())
+                                    .color(Color::Muted)
+                                    .size(LabelSize::Small),
+                            ),
                     ),
             )
     }