settings_ui: Add UI and buffer font weight controls (#15104)

This PR adds settings controls for the UI and buffer font weight
settings.

It also does some work around grouping the settings into related
sections.

Release Notes:

- N/A

crates/ui/src/components/settings_group.rs

@@ -0,0 +1,36 @@
+use gpui::AnyElement;
+use smallvec::SmallVec;
+
+use crate::{prelude::*, ListHeader};
+
+/// A group of settings.
+#[derive(IntoElement)]
+pub struct SettingsGroup {
+    header: SharedString,
+    children: SmallVec<[AnyElement; 2]>,
+}
+
+impl SettingsGroup {
+    pub fn new(header: impl Into<SharedString>) -> Self {
+        Self {
+            header: header.into(),
+            children: SmallVec::new(),
+        }
+    }
+}
+
+impl ParentElement for SettingsGroup {
+    fn extend(&mut self, elements: impl IntoIterator<Item = AnyElement>) {
+        self.children.extend(elements)
+    }
+}
+
+impl RenderOnce for SettingsGroup {
+    fn render(self, _cx: &mut WindowContext) -> impl IntoElement {
+        v_flex()
+            .p_1()
+            .gap_2()
+            .child(ListHeader::new(self.header))
+            .children(self.children)
+    }
+}