Added css variables for breakpoints

[shaack]

See Issue https://github.com/twbs/bootstrap/issues/36094 "css variables for breakpoints are missing in Bootstrap 5".

The css variables for the breakpoints have been missing. I just added them in scss/_root.scss.

Without them I have no idea how to detect the current breakpoint from JavaScript, like I do in my detection script bootstrap-detect-breakpoint which works with BS 4 but not with BS 5 because of the missing css variables.

Furthermore this should be present according to the documentation: "[...]we include our grid breakpoints as CSS variables[...]".