linux: Primary clipboard (#10534)

Implements copying from and pasting to the primary selection.

Release Notes:

- N/A

crates/gpui/src/platform/linux/headless/client.rs

@@ -79,8 +79,14 @@ impl LinuxClient for HeadlessClient {
     //todo(linux)
     fn set_cursor_style(&self, _style: CursorStyle) {}
 
+    fn write_to_primary(&self, item: crate::ClipboardItem) {}
+
     fn write_to_clipboard(&self, item: crate::ClipboardItem) {}
 
+    fn read_from_primary(&self) -> Option<crate::ClipboardItem> {
+        None
+    }
+
     fn read_from_clipboard(&self) -> Option<crate::ClipboardItem> {
         None
     }