Make all HighlightStyle properties optional

Previously, some of those properties such the font weight, style and color
would be mandatory: when the theme didn't specify them, Zed would use a default
value during deserialization. This meant that those default properties would
unconditionally override the base text style, causing a rendering bug when
combining syntax highlights with diagnostic styles.

This commit fixes that by making `HighlightStyle`s more additive: each property
can be set independently and only the properties that theme specifies get
overridden in the base text style.

crates/gpui/examples/text.rs

@@ -62,7 +62,7 @@ impl gpui::Element for TextElement {
                 .select_font(family, &Default::default())
                 .unwrap(),
             color: Color::default(),
-            underline: None,
+            underline: Default::default(),
         };
         let bold = RunStyle {
             font_id: cx
@@ -76,7 +76,7 @@ impl gpui::Element for TextElement {
                 )
                 .unwrap(),
             color: Color::default(),
-            underline: None,
+            underline: Default::default(),
         };
 
         let text = "Hello world!";