Add Sass variable for CSS variable prefix

[mdo]

Suggestion from @8nix at #26596 (comment).

Question though: should we include the trailing - in the prefix variable? Thinking we either do this, or detect if it's nil and remove the dash. Leaning towards the former and requiring that we include it.

Thoughts @twbs/css-review?

[ffoodd]

We may invert your suggestion too, adding the dash if a string is set—here's a SassMeister.

Not sure if it's better or not, just another possible way :)

[MartijnCuppens]

Question though: should we include the trailing - in the prefix variable? Thinking we either do this, or detect if it's nil and remove the dash. Leaning towards the former and requiring that we include it.

I'm ok with the current implementation. We'll need the prefix in other files like https://github.com/twbs/bootstrap/blob/main/scss/_tables.scss too, not sure if there are any other files (read: too lazy to look it up 😄)

[mdo]

Good point @MartijnCuppens, might undo the last change here and keep it with the - in there given the prefix is used throughout the project. Will keep thinking!

[MartijnCuppens]

LGTM

[XhmikosR]

Just a reminder (mostly for myself since I usually go on a merging spree 😁) some PRs target a next release like alpha3.

[XhmikosR]

@mdo this needs a rebase