A single import that brings the client core into scope, along with everything from fivem.shared.Core.

import fivem.client.Core;

class ClientMain {
    static function main() {
        Thread.everyFrame(() -> {
            var vehicle = LocalPlayer.vehicle();
            if (vehicle != null) Ui.drawText('${Math.round(vehicle.speed * 3.6)} km/h', new Vector2(0.5, 0.9));
        });
    }
}

Everything here is a plain type alias, so this costs nothing at runtime. Note that Entity, Ped and Vehicle refer to the wrapper classes in fivem.client.core, not the native externs of the same name in fivem.client.natives — import those explicitly (and preferably aliased) if you need both in one file.