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/settings/src/editable_setting_control.rs

@@ -0,0 +1,33 @@
+use fs::Fs;
+use gpui::{AppContext, RenderOnce, SharedString};
+
+use crate::{update_settings_file, Settings};
+
+/// A UI control that can be used to edit a setting.
+pub trait EditableSettingControl: RenderOnce {
+    /// The type of the setting value.
+    type Value: Send;
+
+    /// The settings type to which this setting belongs.
+    type Settings: Settings;
+
+    /// Returns the name of this setting.
+    fn name(&self) -> SharedString;
+
+    /// Reads the setting value from the settings.
+    fn read(cx: &AppContext) -> Self::Value;
+
+    /// Applies the given setting file to the settings file contents.
+    ///
+    /// This will be called when writing the setting value back to the settings file.
+    fn apply(settings: &mut <Self::Settings as Settings>::FileContent, value: Self::Value);
+
+    /// Writes the given setting value to the settings files.
+    fn write(value: Self::Value, cx: &AppContext) {
+        let fs = <dyn Fs>::global(cx);
+
+        update_settings_file::<Self::Settings>(fs, cx, move |settings, _cx| {
+            Self::apply(settings, value);
+        });
+    }
+}