tui: Patch cell content style instead of overwriting

This is a follow-up to helix-editor#13776 to patch cell contents rather than
overwrite the contents. `ui.text.focus` might only set a modifier like
bold or italic. This should not become the only style in the cell: it
should be layered on top so that the cell has the directory and text
colors from the contents.

Author: the-mikedavis

helix-tui/src/widgets/table.rs

@@ -37,15 +37,14 @@ pub struct Cell<'a> {
 impl Cell<'_> {
     /// Set the `Style` of this cell.
     pub fn style(mut self, style: Style) -> Self {
-        self.style = style;
-        self.content.set_style(style);
+        self.set_style(style);
         self
     }
 
     /// Set the `Style` of this cell.
     pub fn set_style(&mut self, style: Style) {
         self.style = style;
-        self.content.set_style(style);
+        self.content.patch_style(style);
     }
 }