Refactor all_font_names (#15345)

In the current code implementation, it seems that the only difference
between `all_font_names` and `all_font_families` is whether dynamically
loaded font resources are included. Specifically, `all_font_families`
returns the names of all system fonts, while `all_font_names` includes
both the system font names and the dynamically loaded font names. In
other words, `all_font_families` is a strict subset of `all_font_names`.
This is what I observed in my tests on macOS.

<img width="682" alt="截屏2024-07-28 00 49 29"
src="https://github.com/user-attachments/assets/47317c28-0074-49d2-bcfa-052cab13e335">

Related codes:
```rust
let x: HashSet<_> = self.all_font_names().into_iter().collect();
let y: HashSet<_> = self.all_font_families().into_iter().collect();
let only_in_x = x.difference(&y).collect::<Vec<_>>();
let only_in_y = y.difference(&x).collect::<Vec<_>>();
println!("=====================================");
println!("1 -> {:?}", only_in_x);
println!("-------------------------------------");
println!("2 -> {:?}", only_in_y);
```

Release Notes:

- N/A

crates/gpui/src/text_system.rs

@@ -17,7 +17,7 @@ use crate::{
     StrikethroughStyle, UnderlineStyle,
 };
 use anyhow::anyhow;
-use collections::{BTreeSet, FxHashMap};
+use collections::FxHashMap;
 use core::fmt;
 use derive_more::Deref;
 use itertools::Itertools;
@@ -78,18 +78,15 @@ impl TextSystem {
 
     /// Get a list of all available font names from the operating system.
     pub fn all_font_names(&self) -> Vec<String> {
-        let mut names: BTreeSet<_> = self
-            .platform_text_system
-            .all_font_names()
-            .into_iter()
-            .collect();
-        names.extend(self.platform_text_system.all_font_families());
+        let mut names = self.platform_text_system.all_font_names();
         names.extend(
             self.fallback_font_stack
                 .iter()
                 .map(|font| font.family.to_string()),
         );
-        names.into_iter().collect()
+        names.sort();
+        names.dedup();
+        names
     }
 
     /// Add a font's data to the text system.