Enable the clippy::std_instead_of_core lint (#8394)

Prefer `core::` over `std::` where either work

* Part of https://github.com/emilk/egui/issues/5735

examples/external_eventloop/src/main.rs

@@ -1,8 +1,9 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 #![expect(rustdoc::missing_crate_level_docs, clippy::unwrap_used)] // it's an example
 
+use core::cell::Cell;
 use eframe::{UserEvent, egui};
-use std::{cell::Cell, rc::Rc};
+use std::rc::Rc;
 use winit::event_loop::{ControlFlow, EventLoop};
 
 fn main() -> eframe::Result {