Variabilize backdrop class name (#34094)

[romdum]

To resolve #34094, create a new class offcanvas-backdrop which extends modal-backdrop. It helps avoid confusion if an offcanvas and a modal is open and two backdrop are shown.

[GeoSot]

@romdum I did some cleanup in order to help this on track. I hope you don't mind

@twbs/css-review any help on the sccs?

The main cause for now is just to extend the modal-backdrop class, use it on offcanvas-backdrop, (initial idea)

In order to avoid duplication, one Idea is to use a new file backdrop.scss create a new class backdrop and use it to both as extend

[GeoSot]

Js scoped, seems ok

[mdo]

Thanks for the fix lol, I must have been braindead earlier. I pushed another change to fix our z-indexes. I'll make sure to cover this in our highlights for the blog post btw. New z-indexes make more sense now IMO.

[mdo]

I'm on airplane wifi and I screwed up the push, so this was merged to main. Working to at least squash those commits in main and we can iterate in subsequent PRs if needed. Sorry about that folks!

[mdo]

I'm on airplane wifi and I screwed up the push, so this was merged to main. Working to at least squash those commits in main and we can iterate in subsequent PRs if needed. Sorry about that folks!