Read if instrument mode is enabled

[Osmo]

It'd be useful to be able to read if instrument mode is enabled, to conditionally enable some stuff, like using features from debug library, and using exclusive APIs, like on_error and (undocumented as of 2.0.77) LuaGuiElement::actual_size.
It doesn't make much sense to load before all other mods just to define a value in instrument-control.lua and see if it is loaded or not. It also makes it harder for more than one mod to use instrument mode features, even if only one of them needs the functionality of loading before/after other mods.
It'd probably be most useful as a value in LuaHelpers, so it'd be available in data and setting stages, and return a name of the enabled instrument mode mod or nil. But only being available at runtime would also be useful.

[protocol_1903]

+1

[Rseding91]

This was added for the next release.