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/src/elements/label.rs

@@ -214,7 +214,7 @@ mod tests {
             "Menlo",
             12.,
             Default::default(),
-            None,
+            Default::default(),
             Color::black(),
             cx.font_cache(),
         )
@@ -223,7 +223,7 @@ mod tests {
             "Menlo",
             12.,
             *FontProperties::new().weight(Weight::BOLD),
-            None,
+            Default::default(),
             Color::new(255, 0, 0, 255),
             cx.font_cache(),
         )