font_count method (#21314)

# Objective

Splitting off some trivial changes from the text rework.

This adds a method that counts the number of rasterized fonts stored in
`FontAtlasSets`.

# Solution

Return the sum of the lengths of all the sets.

Author: ickshonpe

crates/bevy_text/src/font_atlas_set.rs

@@ -21,11 +21,17 @@ impl FontAtlasSets {
         let id: AssetId<Font> = id.into();
         self.sets.get(&id)
     }
+
     /// Get a mutable reference to the [`FontAtlasSet`] with the given font asset id.
     pub fn get_mut(&mut self, id: impl Into<AssetId<Font>>) -> Option<&mut FontAtlasSet> {
         let id: AssetId<Font> = id.into();
         self.sets.get_mut(&id)
     }
+
+    /// Returns the total number of rasterized fonts across all sets.
+    pub fn font_count(&self) -> usize {
+        self.sets.values().map(FontAtlasSet::len).sum()
+    }
 }
 
 /// A system that cleans up [`FontAtlasSet`]s for removed [`Font`]s