Add .visible_on_hover helper method (#3639)

This PR adds a `.visible_on_hover` helper method that can be used to
make an element only visible on hover.

I noticed we were repeating this similar stanza in a bunch of different
spots:

```rs
some_element
    .invisible()
    .group_hover("", |style| style.visible())
``` 

so it seemed like a nice thing to factor out into a reusable utility.

Release Notes:

- N/A

crates/editor2/src/editor.rs

@@ -9766,8 +9766,7 @@ pub fn diagnostic_block_renderer(diagnostic: Diagnostic, is_valid: bool) -> Rend
                         div()
                             .border()
                             .border_color(gpui::red())
-                            .invisible()
-                            .group_hover(group_id, |style| style.visible())
+                            .visible_on_hover(group_id)
                             .child(
                                 IconButton::new(copy_id.clone(), Icon::Copy)
                                     .icon_color(Color::Muted)