Add methods for exponential decay as a framerate-independent alternative to lerp

[passivestar]

First learned about this when I came across this article and recently Freya reminded us that it's a thing in her great talk

I have to confess every time I want framerate-independent interpolation I have to google the formula because i keep forgetting it. This seems to be useful to have in a game engine considering how commonplace interpolation over time is, and my favorite thing about Godot is that it comes with "batteries included" when it comes to useful math functions. It would probably make sense to have this on all classes that have lerp - vectors, basis, color, etc

This is a function for floats from Freya's slide:

Could be called exp_decay, there's no need for a clever name

[Calinou]

Thanks for the proposal! Consolidating in #9803.

[passivestar]

well glad to see I'm not alone in not being able to remember the formula 😄

[SheepCommander]

YAYYYY

THANK YOU