Remove reloading difference async and sync loading

[TPGamesNL]

Description

The issue in #4246 happens because async reloading is different from normal reloading. In normal reloading, the script is first unloaded, then it is loaded. In async reloading, the script is first loaded, then the old version is unloaded.

This behaviour was chosen because the server does not halt during async reloading and having the script unload first would mean there is a timeframe where the script is not running, and apparently this was unwanted behaviour.

In this PR I changed the async reloading behaviour, so that it unloads scripts first, then loads them (same as sync reloading).
This fixes the issue, but the disadvantage of this is that during script reloading commands or functions in the reloading scripts aren't available.

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Target Minecraft Versions: any
Requirements: none
Related Issues: #4246

[Moderocky]

Would it be possible to add a config option to choose which to use? I can imagine some people will want it the other way (to prevent there being a window when some important function isn't running.)

[TPGamesNL]

Would it be possible to add a config option to choose which to use? I can imagine some people will want it the other way (to prevent there being a window when some important function isn't running.)

While it would be possible, I don't think it's a good idea, as that'd introduce the function reloading bug again if that option is enabled. If there is some important function they want to keep available at all times, they wouldn't even be able to reload it because of the bug.

Technically, the issue this PR introduces is also an issue for sync reloading, but it's only visible if the function call happens during reloading (so probably on another thread).

I could also make an exception for functions by making the new function overwrite the old version, but this would only work for functions. For commands, this wouldn't work. I think it would be better to make all of this stuff unavailable during reloading, but I'd like some opinions on it.

[APickledWalrus]

After some discussion with TP, we're going to incorporate this PR into the Structure API at #4108

While this PR adds the disadvantage that during script reloading commands or functions in the reloading scripts aren't available, we have decided that it would be better for us to fix async script loading for now and focus on enhancing it at a later time.