renderer: implement underline cursor

src/renderer/Metal.zig

@@ -1423,6 +1423,7 @@ fn addCursor(
         .block => .cursor_rect,
         .block_hollow => .cursor_hollow_rect,
         .bar => .cursor_bar,
+        .underline => .underline,
     };
 
     const glyph = self.font_group.renderGlyph(

src/renderer/OpenGL.zig

@@ -1024,6 +1024,7 @@ fn addCursor(
         .block => .cursor_rect,
         .block_hollow => .cursor_hollow_rect,
         .bar => .cursor_bar,
+        .underline => .underline,
     };
 
     const glyph = self.font_group.renderGlyph(

src/renderer/cursor.zig

@@ -9,13 +9,14 @@ pub const CursorStyle = enum {
     block,
     block_hollow,
     bar,
+    underline,
 
     /// Create a cursor style from the terminal style request.
     pub fn fromTerminal(style: terminal.Cursor.Style) ?CursorStyle {
         return switch (style) {
             .bar => .bar,
             .block => .block,
-            .underline => null, // TODO
+            .underline => .underline,
         };
     }
 };