propose new transition variables

[guylepage3]

I'd like to propose the addition of new $transition variables that can be applied to various components.

$transitions-primary:        opacity .15s linear !default;
$transitions-secondary:      all .2s ease-in-out !default;

We are entering a new area of UX dominance where interactions are becoming more and more useful in a designer's tool belt. I feel this is a basic but decent start in for the Bootstrap framework.

[guylepage3]

closing this issue. found an older issue here. #21266