TextStyle::default() ask system for known existing font family

Rather than assuming a specific family exists, try a set of specific
names and if they fail, just grab any old font that the system reports
as existing

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/gpui/src/platform.rs

@@ -343,6 +343,7 @@ pub enum RasterizationOptions {
 
 pub trait FontSystem: Send + Sync {
     fn add_fonts(&self, fonts: &[Arc<Vec<u8>>]) -> anyhow::Result<()>;
+    fn all_families(&self) -> Vec<String>;
     fn load_family(&self, name: &str, features: &FontFeatures) -> anyhow::Result<Vec<FontId>>;
     fn select_font(
         &self,