feat: Add on_script_reloaded callback.

[shanecelis]

Summary

I added an on_script_reloaded function that is called before and after on_script_loaded on reloads. This addresses the issue I filed #419.

I updated the core-callbacks doc with example code. Here's the example code shown there:

mode = 1
function on_script_reloaded(save, value)
    if save then
        print("Before I go, take this.")
        return mode
    else
        print("I'm back. Where was I?")
        mode = value
    end
end

I'm not certain about the name. on_script_reload might be the name I'd choose with a blank slate.

Alternatives

One could designate a special variable to capture. But I think a function is preferable because a reload can interrupt at any time and keeping that value up to date all the time instead of when it's required seems inefficient.

One could break this into two functions, but they don't do anything unless both are implemented so it seemed wise to keep them as a single function.

Testing

I exercised it manually with errors in both the saving of state and restoring state, and both were reported as the usual mechanism.

Mistake

I started this branch on top of my other pull request #418. Oops. Luckily #418 is not controversial or large.

[shanecelis]

A further alternative

I guess as an alternative going with the two functions, one could do this:

mode = 1
function on_script_reload()
    print("Before I go, take this.")
    return mode
end

function on_script_loaded(value)
    if value then
        print("I'm back. Where was I?")
        mode = value
    end
end

My criticism above doesn't apply because on_script_loaded() is useful without on_script_reload(). We'd then only have this set of function calls when reloading:

1. on_script_loaded()
2. INITIATE RELOAD
3. value = on_script_reload()
4. CONTEXT CLEARED
5. on_script_loaded(value)

The preceding design looks like this for comparison:

1. on_script_loaded()
2. INITIATE RELOAD
3. value = on_script_reload(true)
4. CONTEXT CLEARED
5. on_script_loaded()
6. on_script_reload(false, value)

[shanecelis]

NOTE: This currently only works when context sharing is enabled. I made an attempt to support non-shared contexts, but it seemed potentially infeasible. I'd love to hear @makspll's take on whether that seems right.