Fix some compiler errors in bevy_easings

[undinococo]

Adding the plugin to cargo.toml would cause the project to not compile, now it does.

[mockersf]

Using #![feature(float_interpolation)] means it wouldn't work on stable anymore

[undinococo]

I reinstalled my OS and now it works, so the problem was on my end. Sorry!

[Carlton-Perkins]

It would appear that I have the same problem that this PR was trying to fix. I suspect the problem is with the nightly compiler and the fact that they are adding a built in lerp function on fNN's soon which prevents calling the function from the interpolation library you are using.

Rust fNN lerp feature request
A good fix for this might be to conditionally compile depending on if the std has already brought in a lerp function, otherwise bring in the one from interpolation

@mockersf LMK if there is anything I can do to help fix this!