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/gpui/src/text_system.rs

@@ -64,7 +64,7 @@ impl TextSystem {
             fallback_font_stack: smallvec![
                 // TODO: This is currently Zed-specific.
                 // We should allow GPUI users to provide their own fallback font stack.
-                font("Zed Mono"),
+                font("Zed Plex Mono"),
                 font("Helvetica"),
                 font("Cantarell"), // Gnome
                 font("Ubuntu"),    // Gnome (Ubuntu)