Move from Zed fonts to IBM Plex (#13596)

Release Notes:

- Changed the default fonts to IBM Plex Sans and IBM Plex Mono. If you'd
like to go back to using Zed Sans/Zed mono you need to
[download](https://github.com/zed-industries/zed-fonts/releases/tag/1.2.0)
them and install them.

crates/assets/src/assets.rs

@@ -52,4 +52,13 @@ impl Assets {
 
         cx.text_system().add_fonts(embedded_fonts)
     }
+
+    pub fn load_test_fonts(&self, cx: &AppContext) {
+        cx.text_system()
+            .add_fonts(vec![self
+                .load("fonts/plex-mono/ZedPlexMono-Regular.ttf")
+                .unwrap()
+                .unwrap()])
+            .unwrap()
+    }
 }