Unify `Die` behavior of initial / cloned sprite

[nighca]

In SPX, there are different implementations of the Die method for initial and cloned sprites:

https://github.com/goplus/spx/blob/3df776b555a0532999520609e172f45c1816de49/sprite.go#L483-L494

Ideally, there should be no noticeable differences for game developers, but this is not the case. For example, consider this sprite code:

onStart => {
    for {
        wait 0.1
        println "im alive"
    }
}

onStart => {
    wait 1
    die
}

After calling die, the loop continues, so "im alive" keeps being printed.

In contrast, for a cloned sprite:

onCloned => {
    for {
        wait 0.1
        println "im alive"
    }
}

onCloned => {
    wait 1
    die
}

onStart => {
    clone
}

After calling die, the loop stops, and "im alive" is no longer printed.

Game developers may perform more complex actions than just println "im alive", potentially affecting overall game behavior. Therefore, it is crucial to unify the behavior of die for both sprite types.

Please check this out, @xushiwei @JiepengTan.

[nighca]

Based on the comment here

In Scratch and SPX, the initial instance of a sprite is the prototype (somewhat similar to JavaScript). The prototype instance cannot be destroyed, so the 'die' function behaves differently for prototype instances and cloned instances.

@xushiwei To address compatibility concerns, I suggest:

• Rename current destroy to deleteThisClone for compatibility with Scratch's Delete This Clone
• Introduce new destroy, which always destroys the current instance, regardless of whether it's the initial or a cloned instance
• Implement die based on destroy, so it always destroys the current instance as well

[nighca]

closed by #329