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/keyboard_layout.rs

@@ -5,7 +5,7 @@ use std::{
     sync::Mutex,
 };
 
-use once_cell::sync::Lazy;
+use crate::utils::Lazy;
 use smol_str::SmolStr;
 use windows_sys::Win32::{
     System::SystemServices::{LANG_JAPANESE, LANG_KOREAN},