FTI - Add reset on setting top_level

[lawnjelly]

Fixes #108086

I've labelled 4.5, but it's really up to production whether to push this to 4.6 as it is easily get aroundable, and is more an enhancement than bug fix, so feel free to push to 4.6.

Notes

• Although reset_physics_interpolation() will cure this (as with any teleport), I think this should be fine for adding an auto-reset, I can't think of any situation where you'd want to toggle top_level and not perform a reset.
• I've taken the opportunity to check the new SceneTree::is_fti_enabled() static function to make reset_physics_interpolation() super cheap to call when FTI is off.
• Also applicable to 3.x.

[rburing]

I agree this behavior is desired in 100% of cases.

[Repiteo]

Maybe this is technically an enhancement, but in practice this is in-line with other bugfixes we've merged this cycle, so I'm comfortable including it

[Repiteo]

Thanks!