Introduce reveal_path in Platform

And implement it for MacPlatform, and instead of using an external process to run `open`, use the NSWorkspace selectFile instance method.

crates/gpui/src/platform/test.rs

@@ -173,6 +173,8 @@ impl super::Platform for Platform {
 
     fn open_url(&self, _: &str) {}
 
+    fn reveal_path(&self, _: &Path) {}
+
     fn write_credentials(&self, _: &str, _: &str, _: &[u8]) -> Result<()> {
         Ok(())
     }