Add .SystemUIFont to font list (#15340)

As discussed in #15326, this font name should be included in the font
list since `settings.json` indicates that one can set the font to
`.SystemUIFont`.

Release Notes:

- N/A

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/gpui/src/text_system.rs

@@ -84,6 +84,7 @@ impl TextSystem {
                 .iter()
                 .map(|font| font.family.to_string()),
         );
+        names.push(".SystemUIFont".to_string());
         names.sort();
         names.dedup();
         names