Add IsValidPreciseTimer native

[Katzu09]

Summary

This PR adds a new native function IsValidPreciseTimer(timerid) that makes it possible to test whether a specified timer ID is still active and valid.

It also fixes a problem where callback functions continue to execute even after DeletePreciseTimer(timerid) is executed within the callback itself.

Usage Example (PAWN)

new MyTimer;

public OnPlayerConnect(playerid)
{
    MyTimer = SetPreciseTimer("HelloTimer", 2000, true, "i", playerid);
}

public HelloTimer(playerid)
{
    // check if it is still valid before deleting
    if(IsValidPreciseTimer(MyTimer))
    {
        DeletePreciseTimer(MyTimer);
        printf("Timer stopped for player %d.", playerid);
    }
}