[3.x] FTI - Change SceneTree global setting to static

[lawnjelly]

Also fixup FTI configuration warnings so that they only output when the project is using FTI.

Backport of #107886.

@BastiaanOlij noticed that some FTI configuration warnings were firing when FTI was off in the project.
This happened because checking whether FTI was on in the editor was previously unsupported (as it was hard coded to OFF).

I had also been long term intending to swap using a local variable for global on / off in SceneTree to a static, the reason being that it would no longer be necessary to check is_inside_tree() and access the variable via a pointer, for what is likely to be a bottleneck check (and the global setting hardly ever changes, so a static makes sense here).

So I take the opportunity to swap both the global setting to a static, and add an extra one for checking the on / off setting within the editor (while not actually enabled the FTI functionality).

This enables us to check the project global setting before issuing configuration warnings, without having to use e.g. GET_GLOBAL_CACHED(), which isn't a particularly good way of doing this here due to the string duplication.

[lawnjelly]

Thanks!