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.
While there's a separate event to deliver modifiers for keyboard,
unfortunately, it's not even remotely reflects the modifiers state.
Thus use events along side regular modifier updates to correctly
detect the state. Also, apply the modifiers from the regular
key event by converting their state to xkb modifiers state.
Links: https://github.com/alacritty/alacritty/issues/7549Closes: #3388
