gpui: Make screen capture dependency optional (#32937)

Add `screen-capture` feature to gpui to enable screen capture support.  The motivation for this is to make dependencies on scap / x11 / xcb optional.

Release Notes:

- N/A

---------

Co-authored-by: Michael Sloan <michael@zed.dev>

crates/gpui/src/platform/test/platform.rs

@@ -263,10 +263,12 @@ impl Platform for TestPlatform {
         Some(self.active_display.clone())
     }
 
+    #[cfg(feature = "screen-capture")]
     fn is_screen_capture_supported(&self) -> bool {
         true
     }
 
+    #[cfg(feature = "screen-capture")]
     fn screen_capture_sources(
         &self,
     ) -> oneshot::Receiver<Result<Vec<Box<dyn ScreenCaptureSource>>>> {