linux: Primary clipboard (#10534)

Implements copying from and pasting to the primary selection.

Release Notes:

- N/A

crates/gpui/src/platform/mac/platform.rs

@@ -849,6 +849,8 @@ impl Platform for MacPlatform {
         }
     }
 
+    fn write_to_primary(&self, _item: ClipboardItem) {}
+
     fn write_to_clipboard(&self, item: ClipboardItem) {
         let state = self.0.lock();
         unsafe {
@@ -886,6 +888,10 @@ impl Platform for MacPlatform {
         }
     }
 
+    fn read_from_primary(&self) -> Option<ClipboardItem> {
+        None
+    }
+
     fn read_from_clipboard(&self) -> Option<ClipboardItem> {
         let state = self.0.lock();
         unsafe {