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/gpui/src/text_system.rs

@@ -595,6 +595,19 @@ impl FontWeight {
     pub const EXTRA_BOLD: FontWeight = FontWeight(800.0);
     /// Black weight (900), the thickest value.
     pub const BLACK: FontWeight = FontWeight(900.0);
+
+    /// All of the font weights, in order from thinnest to thickest.
+    pub const ALL: [FontWeight; 9] = [
+        Self::THIN,
+        Self::EXTRA_LIGHT,
+        Self::LIGHT,
+        Self::NORMAL,
+        Self::MEDIUM,
+        Self::SEMIBOLD,
+        Self::BOLD,
+        Self::EXTRA_BOLD,
+        Self::BLACK,
+    ];
 }
 
 /// Allows italic or oblique faces to be selected.