Move PopoverButton into ui (#25724)

This PR moves the `PopoverButton` component into the `ui` crate.

The `popover_button` crate only depended on `ui`, so there doesn't seem
to be a need for it to live in its own crate and add another step in the
crate graph.

Release Notes:

- N/A

crates/language_model_selector/Cargo.toml

@@ -17,7 +17,6 @@ gpui.workspace = true
 language_model.workspace = true
 log.workspace = true
 picker.workspace = true
-popover_button.workspace = true
 proto.workspace = true
 ui.workspace = true
 workspace.workspace = true