Add .d-flex utility class for display:flex

[atu-record]

I'd suggest to add display:flex to utilities
this will be need a lot i guess.

[cvrebert]

Do you mean "add one class that just sets display:flex", or something else?

[atu-record]

exactly - same as the .d-block in _display.scss
a .d-flex would be helpful so we can use the flex aligment classes (_flex.scss) easier outside the grid when flex is enabled.

[atu-record]

playing around with flex - i'd realised it would also be helpfull to have responsive display classes
something like

.d-flex-xs
.d-flex-sm
.d-flex-lg
.d-flex-xl

[cvrebert]

I expect that would probably get rejected, per #18410.

[mdo]

Adding a flex utility is something we can do I think, but definitely conditional to flexbox mode and what not.

[mdo]

This was added in #20934.