m: Remove once_cell dependency

Removes the once_cell dependency, instead using std::sync::OnceLock and a
minimal polyfill for std::sync::LazyLock, which may be stabilized soon
(see rust-lang/rust#121377).

This should not require a bump in MSRV, as OnceLock was stabilized in 1.70,
which this crate is using.

src/platform_impl/windows/dark_mode.rs

@@ -2,7 +2,7 @@
 /// which is inspired by the solution in https://github.com/ysc3839/win32-darkmode
 use std::{ffi::c_void, ptr};
 
-use once_cell::sync::Lazy;
+use crate::utils::Lazy;
 use windows_sys::{
     core::PCSTR,
     Win32::{