add the fade transition to a css var

[xvaara]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

we have var(--bs-offcanvas-transition); so could we have a global var(--bs-fade-transition); also?

Motivation and context

We need to jump through a lot of hoops to support bootstrap transitions in boostrap-vue-next, and with that var we could simplify by creating our own classes and offer custom transitions after that.