gpui: Impl Default for TextRun (#41084)

Release Notes:

- N/A

When I was implementing Input, I often used `TextRun`, but `background`,
`underline` and `strikethrough` were often not used.

So make change to simplify it.

crates/gpui/src/text_system.rs

@@ -739,7 +739,7 @@ impl Display for FontStyle {
 }
 
 /// A styled run of text, for use in [`crate::TextLayout`].
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, Default)]
 pub struct TextRun {
     /// A number of utf8 bytes
     pub len: usize,
@@ -813,6 +813,12 @@ pub struct Font {
     pub style: FontStyle,
 }
 
+impl Default for Font {
+    fn default() -> Self {
+        font(".SystemUIFont")
+    }
+}
+
 /// Get a [`Font`] for a given name.
 pub fn font(family: impl Into<SharedString>) -> Font {
     Font {